Part 8

The vertically expanded anterior part of the premaxillary (ib. fig. 1, 22) has a large pneumatic cavity communicating by a reticulate wall with the cells of a cancellous structure, larger than those of the cranial diploë. The maxillary branch of the premaxillary (ib. 22*) consists of a light open-work air-diploë, with a very thin outer case of bone. The short symphysis mandibulæ shows a small cavity, surrounded by more minutely cancellous structure and thicker compact walls, especially at the upper and hinder parts.

Although some characters have been too much insisted on (e.g. the “superoccipital foramen”) as exemplifying the affinity of the Dodo, the more essential characters of the skull relate to its true Columbine character, while the deviations from that part of the skeleton of volant Doves are explicable in the adaptive developments needed for the wielding of long, powerful, massive mandibles, serving most probably to enable the bird to subsist on some proportion of animal diet, in addition to such vegetable food as it might gain from the ground. Such indiscriminate feeding doubtless rendered its flesh less palatable than that of the winged Pigeons of the Mauritius to the Dutch navigators of the sixteenth and seventeenth centuries.

But the affinities of Didus will be more fully and decisively brought out in the comparison of the, in this respect, more instructive and light-giving parts of the skeleton.

§ 3. Comparison of the Skeleton.

The dorsal region of the vertebral column shows, in some birds, a confluence of certain vertebræ: I have observed four to be so welded together by both centrums and neural spines in Phœnicopterus, viz. the second to the fifth dorsal inclusive, leaving the sixth free, which articulates with the first costigerous sacral vertebra. In Platalea three dorsals coalesce in advance of the antepenultimate free vertebra. In the smaller diurnal birds of prey five dorsal vertebræ are usually confluent, leaving one free vertebra for the lateral movements of the trunk between such dorsal “sacrum” and the pelvic one. In Vultures, Plovers, Bustards, Cranes, Psophia, Cariama, Palamedea, Auks, Penguins, and in all flightless land-birds save the Dodo, no such anchylosis takes place. The Columbidæ are the species in which the dorsal vertebræ, homologous and the same in number with those of Didus, undergo the process of confluence into one mass of bone: they are the three which immediately precede the last (moveable) dorsal vertebra; and of these the two anterior develope, in Goura and Didunculus, hypapophyses closely corresponding in shape and proportion with those in the Dodo.

The chief difference which Didus offers in the present region of the vertebral column from that of Columbidæ is in the greater number of the vertebræ or segments which are typically completed by bony hæmapophyses articulating with pleurapophyses and directly with their mass of coalesced and expanded hæmal spines constituting the sternum. Of these typical thoracic segments there were five in Didus (Pl. III.); Didunculus (ib.) shows four; Goura three. In both existing genera these segments are succeeded by a single one, anchylosed to the fore part of the sacrum, but with the pleurapophysis long and moveable, with its hæmapophysis terminating in a point before reaching the sternum, and extensively connected with the antecedent hæmapophysis or sternal rib: in both genera two dorsal vertebræ in advance of the typically complete one have moveable pleurapophyses terminating freely in a point, with no hæmapophyses other than the costal processes of the sternum may represent. In Goura, which has six pairs of moveable or thoracic ribs, the second pair belong to the first of the three anchylosed dorsal vertebræ: in Didunculus, which has seven pairs of thoracic ribs, the second pair belongs to the free dorsal immediately in advance of the anchylosed mass. Supposing Didus to have had one pair of ribs behind, and two pairs in front of those that directly articulate with the sternum, as the vertebra Pl. V. fig. 7 indicates, it would have had eight pairs of thoracic ribs; and I think this excess of one pair beyond the formula in Didunculus to be very probable in the large-bodied, small-winged, extinct Ground-dove.

As far as the series of Dodo’s neck-vertebræ under my observation exhibit such characters, the proportion of those with neural spines, or with hypapophyses, or both, is the same as in the Columbidæ. In this family, as in most birds, the greater part of the series want both processes. The cervical parapophyses, descending to form the sides of the carotid canal, do not meet, coalesce, and circumscribe it in any cervical vertebra of Goura or Didunculus; and not any of the vertebræ of Didus, which I have yet received, shows such circumscription of the hæmal canal. The majority of the cervicals in Didus (those, viz., that lack both neural spines and hypapophyses) are broader and more massive in proportion to their length than in the winged Doves. The third cervical in Didus has both the above processes, as in Columbidæ: the characters of the axis vertebra in the same family are closely repeated in that of the Dodo. In the Raptores the axis vertebra is shorter in proportion to its length, and a greater proportion of the cervical vertebræ at both ends of the series have both neural spines and hypapophyses.

The ribs of the Dodo are as broad, in proportion to their length, as in Doves, but are relatively longer in proportion to the dorsal region, encompassing a more capacious thoracic-abdominal cavity. The ribs of the Vulture are more expanded than in Didus, especially where they afford the extensive attachment to the epipleurals. But I shall not dwell further on the comparative characters of this part of the skeleton, as more decisive ones of the affinity of Didus are afforded by other parts.

In comparing the sternum of the Dodo with that of Doves of flight, the first well-marked difference is in the adaptive development of the keel in the last (Pl. III. fig. 2, Didunculus), and in the provision for the concomitantly broader coracoids, the grooves for which meet and run into each other across the fore part of the bone in existing Columbidæ (Pl. XII. fig. 2, b); consequently the inner or upper wall of the confluent grooves forms a median prominence (ib. e) at the front margin of the sternum, contrasting with the wide notch at that part of the bone in the Dodo (Pl. IV. fig. 4). The next difference, as compared with Goura and most Pigeons, is the absence of the entolateral processes (Pl. XII. fig. 3, i) in the Dodo’s sternum: but Didunculus singularly exemplifies its nearer affinity to Didus by a like absence of those processes; only the sternal margins behind the ectolateral processes (ib. fig. 1, h), instead of converging with a slight convexity to an obtuse apex, as in Pl. VI., describe a concavity, through an expansion of the posterior truncate end of the breast-bone. The sternum of Didunculus may be said to show one pair of posterior notches (Pl. XII. fig. 1, f), that of other Pigeons two pairs (ib. fig. 3, f f′); but the sternum of Didus, which is relatively broader, shows no other trace of the anterior notch (Pl. VI. f) than is afforded by the rounded angle at which the ectolateral process (h) rises from the bone. Although the costal margin is relatively shorter in Doves of flight than in the Dodo, again an intermediate condition is manifested by Didunculus as compared with Goura, in which latter Dove there are articular surfaces for three sternal ribs (Pl. XII. fig. 3, o 1, 2, 3), whilst in Didunculus there are four (ib. fig. 1, c). Didunculus also exhibits, more strongly than Goura, the obtuse ridges (ib. fig. 2, r) converging like buttresses from the outer wall of the coracoid groove to the fore part of the keel, where they subside. In Didunculus there is a pneumatic foramen exterior to the coracoid groove, corresponding with p, fig. 4, Pl. IV., which I do not find in the sternum of Goura; but in the Crown-pigeons the pneumatic foramina along the middle line of the upper surface of the sternum are conspicuous; they are confined to the fore part of that surface in Didunculus (Pl. XII. fig. 1).

In the direction of the ectolateral processes Goura (ib. fig. 3, h) is intermediate between Didunculus and Didus. The pectoral ridge on the outer surface of the sternum, continued backward from the outer end of the coracoid groove, is adaptively better marked in Pigeons of flight than in the Dodo; and the pair of ridges are more nearly parallel in their backward course, not so convergent as in Didus. In Goura the subcostal ridge is better marked than in Didunculus. In no Dove of flight is the body of the sternum so broad and hollow as in Didus (Pl. XI. fig. 4); in this respect the Vulture more nearly resembles the Dodo, as it does also in the more convex anterior contour of the keel: but the vulturine sternum does not lose breadth as it extends backward; it is a square-shaped shield in birds of prey, shorter in proportion to its breadth, with a greater extent of costal process and margin, and with the ectolateral processes, when they exist, extending backward as far as the hinder border of the bone. In the thorough quest of resemblances to the Dodo’s sternum which I have made through the class of Birds, I came upon an unexpected superficial likeness to it in the sternum of a Night-jar (Podargus humeralis). The ectolateral processes (Pl. XII. fig. 4, h) rise behind the moderately extended costal borders, c; and beyond them the body of the sternum converges to an obtuse end, with a contour similar to that in Didus. Moreover the coracoid grooves are divided from each other by a free concave border, less deep and extensive, indeed, than in Didus, but as free from any trace of episternal projection. The ectolateral processes, however, are extended backward to beyond the sternal body; and this part usually shows a pair of small entolateral notches, f′, of which one was present on one side in the specimen figured.

Through the reduction of the coracoids in all flightless birds, there is an interval between their sternal articulations: this is long and concave in the Dodo, but is longest and most deeply concave in Apteryx; it is long but almost straight in Rhea; in Casuarius and Dromaius it is narrow but deeply notched; in Struthio it developes a short episternal process. In no Grallatorial sternum with both ecto- and ento-lateral processes (as e.g. Otis, Œdicnemus, Charadrius) do the former project, as in Didus and the Rasores, immediately behind the costal margin, but they are continued, parallel with the keel, from the outer and posterior angle of the sternum, distant from the costal margin. In old Plovers the entolateral process joins the contiguous angle of the sternal body, and converts the inner notch into a foramen.

In the breast-bone of the Dodo we plainly discern the Columbine modification of the Gallinaceous type, simplified in the minor development of those parts relating adaptively to the power of flight, and expanded and excavated for the support of the larger gizzard with its heavier grindstones.

In comparing the pelvis of Didunculus and Goura (Pl. XII. fig. 5) with that of Didus (Pl. VII. fig. 1), the correspondences are:—in the general shape, proportions and disposition of the ilia; in the articulation therewith of the last pair of moveable ribs, and of the short straight confluent pleurapophyses of the three succeeding sacral vertebræ; then follow, as in Didus, three vertebræ without pleurapophyses, these reappearing in the next two with their extremities converging to abut against a prominence of the inner surface of the ilium in the same relative position. The difference here is in the two equal and more slender rib-buttresses, in place of the single stronger one, which is the more common structure in Didus; but in Goura I have noted an instance in which it agreed with the Didunculus on the left side, and with Didus on the right, in the last-specified character. In the Crown-pigeons, also, there is an indication of the transverse ridge marking off the under part of the centrum of the first sacral from the rest, and those that follow are less expanded than in the Dodlets; moreover in Didunculus they show a median canal instead of a ridge, while the ridge is feebly indicated here and there and there is no canal in Goura. In neither Didunculus nor Goura do the sacral centrums behind the last rib-abutments diminish in breadth so suddenly as in Didus: in both the winged Pigeons the hinder part of the pelvic cavity is relatively deeper and narrower than in Didus; in both, also, the upper and anterior concave tracks of the ilia are deeper; and in Didunculus the mesial borders do not attain the neural crest, but leave a pair of open longitudinal canals at that part of the pelvis; in Goura those margins reach the neural crest, but do not overtop it at any part. In Goura the acetabula are more in advance of a median position than in Didunculus, Columba magnifica, or Didus. Although the ischiadic foramina are completed by terminal confluence of the ilium and ischium in Dromaius and Casuarius, yet the length of those foramina (which are unclosed) in Struthio and Apteryx, concomitant with the greater relative length of the pelvis, shows the difference of Didus from the cursorial Brevipennates in this part of the skeleton. The ischia of the winged Pigeons resemble those of the Dodo; but the inner longitudinal ridge is more strongly marked in Didunculus: in the Goura it is less developed than in Didus; the bone is longer also in proportion to its breadth, and the ischiadic foramen is longer and narrower: the proportions of that in Didunculus are more like those in Didus. In Didunculus the pubis coalesces with the ischium behind the small obturator foramen, but leaves a second or posterior elongate ischio-pubic vacuity. The greatest amount of resemblances with the pelvis of the Dodo is found in that of different members of the Dove-tribe.

In comparing the pelvis of the Dodo with that of the Vulture (Pl. XII. fig. 6), we find in the latter that the first two confluent sacral vertebræ; supporting moveable ribs are succeeded by several with short abutting ribs, the extent of this part of the sacrum being nearly one-half of the whole, instead of one-fourth as in Didus and the Doves. The reappearance of rib-abutments after four ribless sacrals is in the posterior third of the sacrum, and they are continued to the end of that bone from the last four vertebræ of the series, constituting a very marked difference, both as to number and the character of the vertebræ in the sacral part of the pelvis.

With regard to the iliac bones, the anterior concave track occupies two-thirds of the extent of the bone in Vultur, not one-half as in Didus and most Doves; the breadth of the posterior parts of the ilia with the intervening sacrum in the Vulture is relatively less than in the winged Doves, and differs in a greater degree from that characteristic part in the sacrum of Didus. In Ciconia the antacetabular part of the pelvis is relatively longer, and the iliac bones are more expanded anteriorly. In Platalea the proportions are more nearly those in Didus. In Otis the ilia touch the fore part of the sacro-spinal ridge, but leave both posterior and anterior apertures of the ilio-neural canals widely open. In Œdicnemus and Charadrius they are grooves, the ilia not reaching the sacral spines. The external concavity of the ilium is longer, narrower, and deeper, in most waders, than in Didus. In Eudyptes and Aptenodytes the ilia are more expanded anteriorly, but the whole pelvis is narrower and longer than in Didus. The Gar-fowl (Alca impennis), Uria, Podiceps, and Colymbus, all show still longer and narrower proportions of the pelvis.

In the Doves of flight the proportions and relative position of the three compartments of the cranial cavity differ from those in the Dodo. Both the pros- and mes-encephalic ones are proportionally larger than the epencephalic; and the mesencephalic compartment lies more directly below the prosencephalic one. A very thin stratum of finely cellular diploë divides the two tables of the skull along the medial line of the upper surface: it is thicker between the orbits. The falcial ridge at the inner surface of the prosencephalic roof resembles that in Didus. The tentorial ridge bifurcates halfway down, the front portion dividing, almost horizontally, the pros- from the mesencephalic compartment, the hinder and more obtuse ridge dividing, almost vertically, the mes- from the epencephalic compartment. The angle of bifurcation is slightly produced and obtuse, but represents very feebly the tentorial tuberosity (Pl. XI. fig. 1, o) in the Dodo: from it, in Goura, is continued backward the arch of bone formed by the superior semicircular canal, above which is the groove for the venous sinus, as in Didus. The internal auditory fossa is less deep than in Didus: above it is a similarly vertically oblong cerebellar pit. The nerve-foramina correspond with those in Didus: the entocarotid canal opens into a rather deeper sella in Columba palumbus.

On comparing the cranial cavity, as exposed by a vertical longitudinal section in the Dodo (Pl. XI. fig. 1), with that of a Dinornis similarly exposed, the first difference is the smaller proportional depth of the diploë in the larger wingless bird, which is not greater over the prosencephalic than over the epencephalic compartment; next may be noticed the larger relative size of the former compartment, indicating the larger cerebrum of the Dinornis, then the absence of the tentorial tuberosity, the sharper and more produced superior part of the tentorial ridge arching transversely between the cerebrum and cerebellum, the smaller internal auditory fossa, and the deeper sella: the mesencephalic compartment, or cavity for the optic lobe, is less in proportion to the prosencephalic compartment than in Didus; it holds, however, a similar relative position: finally, the cerebellar pit, above the internal auditory fossa, is wanting in the Dinornis.

The Dodo agrees with the Doves in possessing a slender furculum, forming an acute angle: it resembles Columba galeata, more especially, in the halves of that bone being united by ligament below, and forming separate styles or “clavicles.”

The humerus of the Goura closely repeats most of the characters described in that of the Dodo; but its length is proportionally greater, being 3 inches 9 lines, nearly equal to that of the sternum or pelvis, whereas the humerus of the Dodo is little more than half the length of either sternum or pelvis. The processes for the attachment of the muscles are, nevertheless, fully as strongly developed in Didus (Pl. VIII. figs. 12 & 14) as in the volant Doves (Pl. XII. figs. 8 & 9, Goura); that, indeed, which is a ridge (r) on the back part of the shaft in Didus, is a mere rough surface in Goura, and does not show in Didunculus. The pneumatic fossa, which varies in depth in the two humeri of the Dodo, is in both relatively larger and shallower than in Goura. The pectoral process is thinner, but relatively rather more produced, in Didunculus. The humerus in Œdicnemus, Otis, and Charadrius has a more longitudinally extended, thinner, and more produced pectoral ridge than in Didus and the Columbidæ; there is a more marked ectocondyloid tuberosity, which in Charadrius becomes a pointed process.

There is nothing to be gained by giving the details of the more striking differences which the humerus presents in Penguins, Auks, and birds of prey, as compared with that bone in the Dodo; but a few words may be recorded of the comparison of the humerus of the Dodo with that of the flightless bird of New Zealand so nearly approaching to it in size, which bird is described in the 5th volume of the ‘Transactions’ of the Society under the name of Cnemiornis (p. 395, pl. 66. figs. 7–10). In that extinct species, although the humerus is 5½ inches in length, the parts indicative of the forces by which it was worked are comparatively feebly developed. The ulnar tuberosity is narrower, thicker, more obtuse, and its base has neither the upper nor lower excavation; it rises above the articular head, which is less prominent and narrower than in Didus; the pectoral ridge is shorter and situated lower down upon the shaft, not on the same level with the radial tuberosity as it is in Didus; the distal articulation is of the same size as in Didus, but neither the radial nor the ulnar convexity is so prominent or well-defined.

The ulna of the Dodo is shorter absolutely, and much more so proportionally, than in the Goura and most other volant Doves. In these it exceeds the humerus by about one-fourth its own length; in Didunculus (Pl. III.) it is a little longer than the humerus; in the Dodo (ib.) it is shorter than the humerus. The length of the ulna in Goura coronata is 4 inches 6 lines; it is more bent than in the Dodo; the quill-tubercles, seven or eight in number, are more prominent; nevertheless the rough depression for the insertion of the chief flexor is less deep and less defined. The plumed winglet of the Dodo would seem, therefore, to have been frequently and forcibly moved.

In comparing the femur of the Dodo with that of the largest Dove, the bone appears gigantic. The length of the femur in Goura coronata (Pl. XII. fig. 11) is but 3 inches 3 lines, and it is more slender in proportion to its length than in the Dodo; it, however, repeats the few characteristics, if they may be so termed, of the Dodo’s femur. It has the pneumatic foramen in the same position, perhaps proportionally larger; it has the same large oblong surface for the ligament at the head of the bone; the great trochanter has the same form and disposition, but is not quite so much produced anteriorly; there is a slight depression instead of a ridge for the trochanter minor; the fore part of the inner condyle is relatively thicker and less produced. The femur in Otis and Œdicnemus has a thicker and shorter trochanter major, & more narrow and shallow rotular channel; it is shorter in comparison with the tibia, and more especially with the metatarsus, than in Didus and the Doves.

The femur of Aptornis otidiformis is of the same size as that of the Dodo; but it has no pneumatic foramen, the head is more hemispheroid and inclined forward, the ligamentous pit is deeper and more circular, the supracervical articular surface is not defined from that of the head, there is a wider and deeper depression at the fore part of the proximal end of the femur, and a more prominent tuberosity on the back part; the ridge continued from the back part of the shaft to that of the inner condyle is more produced and sharper in Aptornis, the fore part of the same condyle is less produced.

The femur in Cnemiornis and Dinornis is much thicker, in proportion to its length, than in either Aptornis or Didus. In Pezophaps the great trochanterian ridge rises higher above the neck, and the shaft has a more uniform thickness, with the inner contour less concave, than in Didus.

The characters which have been noted at the proximal and distal ends of the tibia of Didus are repeated in those of the tibia of the Goura. The difference in size is more marked than in the femur; the length of the tibia of Goura coronata is 4 inches 7 lines, and its shaft is more slender, in proportion to its length (Pl. XII. fig. 13), than in Didus (Pl. X.). The tendency to a trihedral form of the shaft is less marked in Goura; the anterior prominences of the distal condyles are thicker in proportion to the intervening fossa.

In the Vulture the fibular ridge is more parallel with the long axis of the shaft than in Didus; the tendinal canal is less cylindrical, has an oblique course from the middle of the anterior surface towards the inner condyle; the fore parts of both distal condyles are less produced and less convex; the distal end is narrower from before backwards in proportion to its breadth; both extremities of the bone are less expanded in proportion to the shaft than in the Dodo.

In the great Plover (Œdicnemus crepitans) the tibia, as in other Grallæ, is longer in proportion to its thickness than in Didus; the epicnemial process rises higher above and projects further in front of the condylar surfaces before it divides into the pro- and ectocnemial plates; and these are relatively more produced. The fibular ridge is shorter in proportion to the length of the tibia, is more prominent, and more parallel with the axis of the shaft. The distal condyles project further backward than in Didus. The tibia in Charadrius, Otis, Tantalus, Grus, Ciconia, Mycteria, Porphyrio, opposes similar or equivalent differences to those in Œdicnemus, against the affinity of Didus to any of those Grallæ.

In the comparison of the tibia of this extinct flightless bird with that of the Cnemiornis, the wonderful development of the plates and processes at the proximal end of the bones in the New Zealand bird is strikingly manifested. In Cnemiornis the fibular ridge runs in a line with the shaft, and does not incline from above obliquely forward as in Didus and the Doves; the ridge on the outer side of the distal fourth of the bone is stronger and sharper in Cnemiornis; the tendinal canal is transversely elliptical, medial in position, with a slight inward inclination; the intercondyloid fossa is much wider in Cnemiornis. The differences, indeed, in all the characters of the tibia, as compared with Didus, in the Vultures, Plovers, Penguins, and terrestrial flightless birds tend to render more instructive and convincing the resemblances which Pigeons present in the same characters to the extinct Mauritian bird.