Part 7

There was no carpal or pinion bone in the collection of remains submitted to me: this part of the wing is conjecturally restored in dotted outline in Plate XV.

Bones of the Leg. (Pls. III., IX., X. & XI.)

Of the five femora in the above defined series of remains of the Dodo, two measure 6 inches 3 lines in length; one (Pl. IX.) is 6 inches 4½ lines; the shortest is a little under 6 inches, with proportionate differences in the diameter of the shaft. All of them show a small pneumatic foramen (Pl. IX. figs. 1 & 2, p) on the inner side of the anterior ridge of the great trochanter (ib. c), and on the same transverse line with the head of the bone. This part shows an oblong depression (ib. figs. 2 & 3, a) for the “ligamentum teres” at the upper and back part. The articular surface on the same aspect of the neck (ib. fig. 3, b), adapted to the trochanterian prominence of the pelvis (Pl. VII. t), is well-defined. The trochanter (Pl. IX. fig. 1, c) rises, ridge-like, above the level of the head, and is continued from behind the middle of the articular surface on the neck, forward, with a convex outline upon the fore and outer part of the shaft, where it gradually subsides; a narrow intermuscular ridge (ib. fig. 1, r), inclining to the middle of the fore part of the shaft, is continued from the trochanterian one. The small trochanter (ib. fig. 3, d) is a small subcircular tuberosity, in some specimens a ridge, 3 to 4 lines in length, on the inner side of the shaft, about an inch below the head. The muscular impressions on the fore part of the bone are well defined. A minute medullary canal (ib. fig. 3, m) perforates the middle of the back part of the shaft; the popliteal fossa (ib. fig. 3, o) shows a few small pneumatic orifices; a triangular rough flat surface divides the fossa from the outer condyle. Above the fibular depression (ib. fig. 3, g) there is a well-defined, slightly raised, rough surface (ib. k) for the head of the ectogastrocnemius muscle. The ridge (ib. n) extending to the back part of the inner condyle is not sharp; the rotular groove (ib. fig. 1, p) is deep and moderately wide, with the inner boundary, formed by the narrow anterior part of the inner condyle (ib. fig. 5, e′), most produced. The breadth of this end of the longer femora is 1 inch 9 lines; the character of the distal articular surface is shown in Pl. IX. fig. 5.

The head, neck, and great trochanter (Pl. XI. fig. 6) are occupied by a pneumatic cancellous structure, with a thin compact wall on the upper part and sides: this begins to gain thickness at the under part of the neck and at the lower and back part of the trochanter, the compact wall acquiring a thickness of a line at the beginning of the shaft, where the cancellous structure is confined to the outer side of the pneumatic cavity; this structure gives way to a few delicate filaments of bone crossing the cavity of the major part of the shaft, and is not resumed until the bone expands to form the distal condyles (ib. fig. 7).

The five tibiæ of Didus in the same collection range in length from 8 inches 8 lines to 9 inches. The procnemial ridge (Pl. X. figs. 1, 2, 4, p) is a triangular plate, with the base longest and the apex rounded off: it inclines outwardly, and does not extend much more than half an inch from the level of the proximal end of the bone: the length of its base rather exceeds an inch: on its inner side a triangular muscular surface is well defined by an irregular inferior line or ridge (ib. fig. 2, n). The ectocnemial process (ib. figs. 1, 3, 4, e) is thicker, shorter, and terminates roughly and obtusely. There is a low, narrow ridge (ib. fig. 2, g), about half an inch in length, on the inner side of the proximal end of the shaft, beginning about 9 lines below the articular surface at that end. The fibular ridge (ib. figs. 1 & 3, h), beginning 1 inch 8 lines from the proximal end, extends about 2 inches down the outer side of the shaft. The epicnemial ridge (ib. figs. 1 & 4, k) is obtuse, and but little produced above the upper articular surfaces or condyles (t d) of the tibia: the breadth of that end of the bone, in the longest specimen, is 2 inches 3 lines. The tendinal canal at the fore part of the distal end is bridged by bone (ib. fig. 1, l), and is situated on the inner half of that aspect of the shaft; the lower opening is subcircular and close to the anterior end of the inner lower condyle (ib. a), which is more produced forward than the outer one (ib. b). Their hind ends project very little beyond the level of that aspect of the shaft of the tibia. An intermuscular ridge (ib. fig. 1, r) strengthens into a tuberosity (r′) at the inner side of the tendinal groove.

The cancellous structure in the tibia is limited to an extent of about half an inch below the proximal articular surfaces (Pl. XI. fig. 8), and to about an inch and a half from the distal end of the line (ib. fig. 9): the shaft is occupied by a large air-cavity, with a compact wall of half a line in thickness at the upper third, gradually increasing to about a line at the lower fourth, until the cancellous structure is reestablished; the transverse direction of a plate of this structure indicates the extent of the original distal epiphysis of the tibia (fig. 8).

The fibula (Pl. X. figs. 6–8) presents the usual ornithic characters of the bone: it varies from 4 inches 4 lines to 4 inches 6 lines in length, with a greatest proximal breadth of 8 lines. No adequate gain would result from a detailed description or comparison of this bone; and the rest of the bones of the foot have received every requisite attention in this way in the excellent work on the Dodo and its kindred, already quoted. A longitudinal section of the metatarsus, taken in the direction from side to side (Pl. XI. fig. 10), shows the loose cancellous texture of the common epiphysis of the three long metatarsals, and the remnant of their contiguous coalesced walls reduced to a thin lamella of bone. As the moiety of the bone figured is the posterior one (of the left metatarsus), the usual oblique position of the middle metatarsal (iii), with its proximal end nearer the back part and its distal end nearer the fore part of the coalesced series, produces a corresponding direction of the section, with narrowing and termination of the exposed part of the medullary canal about one-third from the distal end of that metatarsal. The medullary canal of the outer metatarsal (iv) is wider and descends lower before the breaking up of the inner surface into decussating lamellæ or filaments, than that of the inner metatarsal (ii): the peripheral compact wall of the inner is twice the thickness of that of the outer metatarsal. I may remark that the more posterior position of the middle metatarsal at its proximal end, from which and the corresponding part of the common epiphysis the calcaneal process is developed, is related to the greater share taken by the middle toe in the act of walking and scratching. I will only remark that of the four metatarsals of as many Dodos in the present series, one exceeds by a line the length of that figured in plate xi. op. cit., and one falls short thereof to the same trifling amount.

Skull. (Plates III. & XI. fig. 1.)

Of the skull of the Dodo, the series of bones transmitted to me include the cranial part with the detached upper mandibular bone (more or less mutilated) of two mature birds, and the lower mandible of three individuals. In the latter the dentary elements (Pl. XI. fig. 1, 32), confluent at the “gonys,” are distinct from the hinder halves of the rami formed by the confluent, or perhaps connate, articular, surangular and angular elements (ib. 31): if the “splenial” were ever distinct, it has coalesced with the dentary, where its upper boundary is indicated by a linear groove or series of small foramina.

In size, shape, and all other characters of these important evidences of the specific nature of the remains from the Mahébourg morass, they agree with those of Didus ineptus detailed in the ‘Proceedings of the Zoological Society’ for January 11th, 1848 (part xvi. pp. 2–8), and in the work entitled “The Dodo and its Kindred,” pp. 76–96.

The occipital condyle (ib. 1) presents the same hemispheroid or reniform shape, with the median vertical notch or depression above. The upper margin of the foramen magnum is broad, as it were excised, with the sides slightly prominent. The superoccipital foramen is present in both specimens, as in the one originally described (Proc. Zool. Soc. part xvi. p. 2). This foramen also exists in Owls and Parrots, but not in all Pigeons; the Didunculus (Pl. III. fig. 2) shows no trace of it; I have also failed to find it in the skull of a Crown-pigeon (Goura coronata). The superoccipital ridge is defined by the subsidence of the surface beneath it being continued directly from the upper, almost flat, smooth surface of the cranium: the middle part of the ridge is more produced than the angles. In the great breadth of the occipital surface compared with its depth, in its flatness from side to side, and its aspect backward and a little upward, Didus most resembles Dinornis. The basioccipital curves downward, and unites with the basisphenoid in developing the pair of larger tuberosities (Pl. XI. fig. 1, 5), which terminate about ½ an inch below the occipital condyle. There is nothing of this structure in the Columbine cranium. In one of my Dodo’s skulls there is a pair of small tubercles between the larger basioccipital ones; these are not developed in the other cranium. The basisphenoid is subquadrate, and flattish below, impressed by a shallow median longitudinal channel.

The hypoglossal nerve escapes by two small foramina on each side of the base of the condyle; external to these is the vagal foramen; still more external is the depression (ib. a) perforated below by the entocarotid, glossopharyngeal, and sympathetic, above by the tympanic vein. The entocarotid canal opens into the hind part of the sella or pituitary fossa: the vagal canal begins within the skull, above the hypoglossal foramina. The paroccipital carries the posterior surface of the skull downward and outward to a much greater degree than in any Dove, but to a less degree than in Dinornis. The Eustachian tubes impress the outer and fore part of the basisphenoid.

The temporal fossæ (Pl. III.), in the present specimens, show the same contraction in proportion to their depth by which the original skull of the Dodo, compared with that of the Dinornis, ‘Proc. Zool. Soc.’ (1848, p. 3), differed from the larger extinct wingless bird. In the approximation of the postorbital process to the mastoid, Didunculus shows a closer resemblance to Didus than does Goura, in which the temporal fossa, besides being narrow, is shallow. The temporal muscle appears to spread its origin above the fossa upon the sides of the cranium, forward half an inch in advance of the postfrontal process, and backward to the outer angle of the superoccipital ridge.

The parietal region is broad, flat, and short, as in Dinornis, not convex as in Doves; it is also impressed at its middle part by a shallow transverse groove, continued outward and forward of less depth and definition, so as to mark off the convex interorbital part of the swollen frontals.

The outer side of the mastoid is convex, smooth, overhanging the tympanic cavity, and sending off a short process, the base of which is defined in one cranium by a transverse ridge in front of the anterior articular cup for the tympanic bone. A similar process is developed in Didunculus, not in Goura, where it is barely indicated.

The presphenoid is compressed, but thickened and rounded below, where the palatines and pterygoids at their junction with each other abut against it: the pterygoid sends off a short process from the middle of its hinder border; but this is not met by a corresponding “pterygoid process” of the basisphenoid as in Didunculus.

The frontals are broad and convex, rising abruptly (as in Didunculus) above the coalesced cranial ends of the nasals and premaxillary (Pl. III.); in Didus the breadth greatly exceeds the length of the interorbital frontal convexity, as compared with Didunculus, and the convexity reigns in the transverse as well as the antero-posterior direction; in Didunculus, however, it is less concave transversely than in Goura. In the breadth or thickness of the interorbital septum Didus resembles Apteryx and Palapteryx and shows the same pneumatic cancellous structure. The posterior olfactory chambers are partially divided, as in Dinornis, by an upper median septum; each compartment, which is 7 lines across and an inch in length, is perforated posteriorly by an olfactory foramen more than a line in diameter, from which grooved impressions of ramifications of the nerve diverge upon the hind and upper wall of the chamber: external to the olfactory foramen is a longer one for the passage of a vein into the fore and inner part of the orbit.

The cranial ends of the nasals and nasal process of the premaxillary (Pl. XI. fig. 1, 22) are flat, depressed, thin plates; the latter at its junction with the frontal is 6 lines broad, partially divided by a median groove above and a ridge below, and by short linear fissures from the nasals: the forward extension of these bones is feebly indicated by linear grooves terminating at the outer margins of the nasal branch of the premaxillary, about 4 inches from its vertical end. The proportion of the base of the upper mandible attached to the frontal contributed by the nasals is the same as that indicated in the ‘Proc. Zool. Soc.’ l. c. The nasal branch of the premaxillary presents a full elliptical transverse section where it quits the maxillary processes, losing both depth and breadth as it recedes to join the nasals; here it retains its breadth, viz. 6 lines, but continues to be thinned off vertically to the plate above named joining the frontal. The under surface of the narrower part of the stem is angular, the upper one being gently convex.

“Where the nasal and maxillary processes diverge, there is a deep groove externally, terminating in a canal directed forwards into the rostral part or body of the premaxillary”. This part is subdecurved, pointed, roughened by irregular vascular perforations and grooves, with a sharp alveolar border, which describes a sigmoid curve lengthwise, and with a deeper concavity of the palatal surface than in Dinornis or Didunculus. Moreover the concavity is partially divided lengthwise by a median ridge. The palatal surfaces of the maxillary processes and maxillaries are narrow and very convex transversely, intercepting a long narrow palato-nasal fissure. The outer side of the maxillary process is deep vertically and slightly concave lengthwise—a structure not known in Didunculus or any Dove, and related, like most other deviations from the Columbine cranial characteristics, to the provision of unwonted strength of beak in the Dodo. The maxillary branches of the premaxillary have completely coalesced with the maxillaries, as these have with the palatines; and the halves of the upper mandible here swell out laterally and more so vertically, the maxillaries rising to combine with the outer divisions of the nasals, and sending back a short process from their lower and lateral part to join the malar. The inner surface of the maxillary process (Pl. XI. fig. 1, 22*) is smooth and slightly convex vertically; both upper and lower borders are obtuse and thick.

The palatines arch outward from their posterior attachments, are broad and smooth mesially; the margin here is angular, with a slightly produced obtuse apex, divided by a channel on the under surface of the palatine from the outer convex border; the upper and outer ridge extends forward to the maxillary; the inner one subsides before reaching that bone. “The palatines form the posterior boundaries of the naso-palatine aperture, and approximate each other at both ends, but more closely posteriorly, yet here without meeting; whilst in Didunculus they coalesce before receiving the abutment of the pterygoids.

“The tympanic bone is subquadrate, with the four angles produced, and the upper and hinder are bifurcate, forming the double condyle for the mastoid articulation”. There is a larger pneumatic foramen, communicating with the tympanic cavity, between the articulating cavities for these condyles.

The brain is singularly small in the present species of Didus: and if it be viewed as an index of intelligence of the bird, the latter may well be termed ineptus. The length of the cranial cavity (Pl. XI. fig. 1, v c) is 1 inch 8 lines, its extreme breadth 1 inch 6 lines, its greatest height 1 inch (and this is at the cerebellar fossa). The most remarkable feature in the cranial structure of Didus is the disproportionate size of the brain-case to the important part of the neural axis it contained and protected: some approximation to this condition is made by Dinornis, the Owls, and a few large Cockatoos, e. g. Microglossum aterrimum; but it is fully paralleled only by the Elephant among air-breathing vertebrates, as may be seen by comparing the section Pl. XI. fig. 1 with the figures of a similar section quoted below.

Not only was the brain of very small proportional size in the present large extinct bird, but the division of the cranial cavity appropriate to the cerebrum proper is less in proportion to that for the cerebellum and optic lobes, at least in vertical and longitudinal diameters, than in any other known bird.

In the Elephant the thickness of the pneumatic diploë between the fore part of the cerebral cavity and that of the outer cranial wall equals the longitudinal diameter of the cavity containing the cerebral hemispheres: in Didus it exceeds that diameter. The thickness of the pneumatic diploë above the cerebral cavity equals the vertical diameter of that cavity in Didus: the diploë gradually decreases in thickness as it approaches the foramen magnum. The disposition of the osseous lamellæ forming the cells or cavities of the diploë is very different in the Elephant and Dodo: they extend for the most part vertically between the outer and inner tables of the skull in the proboscidian mammal, leaving long and narrow interspaces; in the heavy ground-bird they form a congeries of small subequal and subspherical air-cells, and this structure obtains in the basal and lateral walls as well as in the superior or “roofing” wall of the cranial cavity. The extent of this cancellous structure at the sides of the cranial cavity may be known by the ratio of the breadth of that cavity to the breadth of the cranium, which is 3 inches and 8 lines at the broadest part of the brain, viz. the prosencephalon. It would seem, at first sight, as if the poorly developed brain of the Dodo had needed, on some account, unusual protection; but the true explanation rests on the size, weight, and power of the bill, and the concomitant necessity for adequate extent of attachment of the facial to the cranial part of the skull, and of the muscles from the trunk destined to sustain and wield the long and heavy-beaked head. The cerebrum of the Dodo does not greatly, and by no means proportionally, exceed the size of that part of the brain in the Crown-pigeons (Goura). If the great Ground-dove of the Mauritius gradually gained bulk in the long course of successive generations in that uninhabited thickly-wooded island, and, exempt from the attacks of any enemy, with food enough scattered over the ground, ceased to exert the wings to raise the heavy trunk, then, on Lamarck’s principle, the disused members would atrophy, while the hind limbs, through the increased exercise by habitual motion on land, with increasing weight to support, would hypertrophy.

In the long course of generations subject to this slow rate of change, there would be nothing in the contemporaneous condition of the Mauritian fauna to alarm or in any way to put the Dodo to its wits; being, like other Pigeons, monogamous, the excitement, even, of a seasonal or prenuptial combat, might, as in them, be wanting: we may well suppose the bird to go on feeding and breeding in a lazy, stupid fashion, without call or stimulus to any growth of cerebrum proportionate to the gradually accruing increment of the bulk of the body. Whatever part of the brain was concerned in regulating or controlling muscular actions, might, indeed, be expected to show some concurrent rate of increase with the growing mass of the voluntary contractile fibres; and the size of the cerebellar division (Pl. XI. fig. 1, n o) of the cranial cavity accords with the generally accepted physiology of the superincumbent mass of the epencephalon. The lateral depression at the fore and under part of the side of the postcerebral division of the cranial cavity indicates that the optic lobes, like the eyes, remained almost stationary during the progressive acquisition of the bulk that distinguishes the Dodo from the largest existing Doves.

The proportions of Didus, Pezophaps, Casuarius, Rhea, Dromaius, Struthio, Aptornis, Cnemiornis, Palapteryx, Æpyornis, Dinornis, &c. among terrestrial birds, of Notornis among the lake-haunting Coots, and of Aptenodytes and Alca impennis among seabirds, point to the disuse of wings in flight as the main condition of increase of size in species of birds—the next condition being absence of lethal enemies during the years requisite for such course and rate of growth.

Let foes arise from whom a power of flight is the main condition of escape, and the wingless giants of the feathered class soon succumb. Among the genera above-cited, Aptornis, Cnemiornis, Æpyornis, Palapteryx, Dinornis, Didus, and Pezophaps, with the largest of the Auks, have thus passed away, while Notornis and Apteryx are on the verge of extinction through the rapid increase of population in the small island to which they are restricted. In sparsely peopled continents, such as Africa, South America, and Australia, brevipennate giants may still range the deserts, pampas, and unfrequented wilds. The ascertained recent advent of Man in New Zealand, New Britain, Ceram, Banda, Salwattie, Mauritius, Rodriguez, significantly points to the conditions under which have come to pass, in lapse of time, so strange an anomaly as a bird with the specially modified instruments of flight reduced below the power of exerting that mode of locomotion, yet, as a bird, retaining the conditions of the respiratory and tegumentary systems of the volant class, of which it has become a degenerate member. With the cessation of the chief of those conditions, viz. the absence of enemies, such birds necessarily perish.

Refraining, however, from further indulgence in an easy and seductive vein of speculation, I would recall attention to the notable protuberance in the cranial cavity of the Dodo (Pl. XI. fig. 1, o) developed towards the upper part of the vertical tentorium, contracting at its lower part into the ridge dividing the prosencephalic from the mesencephalic chamber. In the latter are the orifices for the issue of the trigeminal nerve, the larger and posterior (ib. tr) giving passage to the third and second divisions, and answering to the combined foramen ovale and rotundum of mammals, and the smaller and anterior foramen dismissing the first or orbital division of the fifth nerve. At the upper part of the mesencephalic fossa the narrow groove for the lateral venous sinus impresses and defines the back part of the tentorial protuberance, above which it bifurcates, the lower branch bounding or defining the wall of the superior semicircular canal and the upper part of the primitive acoustic capsule. Below this arch is an oblong cerebellar fossa (ib. n) which appears to have received veins from the cranial diploë. Beneath this fossa, and just behind the mesencephalic chamber, is the multiperforate internal auditory depression. Next behind this is the outlet for the vagal nerve and entojugular vein. Below this are the small precondyloid foramina. There is a falcial ridge, low and thick, indicating the division of the prosencephalic chamber into lateral compartments for hemispheres; and this ridge shows a narrow groove as for a small longitudinal sinus. A transverse linear groove abruptly defines the fore part of the ridge.