Part 11

c. Reptile tooth as shown in section.

a, b, & c. Row of ichthyic teeth in dermal plate of jaw.

B. Magnified representatives of ichthyic teeth, a and b, in A.]

It seems truly wonderful, when one considers it, to what minute and obscure ramifications that variety of pattern which nature so loves to maintain is found to descend. It descends in the fishes, both recent and extinct, to even the microscopic structure of their teeth; and we find, in consequence, not less variety of figure in the sliced fragments of the teeth of the ichthyolites of a single formation, than in the carved blocks of an extensive calico print-yard. Each species has its own distinct pattern, as if, in all the individuals of which it consisted, the same block had been employed to stamp it; and each genus its own general type of pattern, as if the same radical idea, variously altered and modified, had been wrought upon in all. In the Dendrodic (Cœlacanth?) family, for instance, it is the radical type, that from a central nave there should radiate, spoke-like, a number of arborescent branches; but in the several genera and species of the family, the branches belong, if I may so express myself, to different shrubs, and present dissimilar outlines. It has appeared to me, that at least a presumption against the transmutation of species might be based on those inherent peculiarities of structure which are thus found to pervade the entire texture of the framework of animals. If we find erections differing from one another merely in external form, we have no difficulty in conceiving how, by additions and alterations, they might be brought to exhibit a perfect uniformity of plan and aspect: transmutation,—development,—progression,—(if one may use such terms,)—seem possible in such circumstances. But if the buildings differ from each other, not only in external form, but also in every brick and beam, bolt and nail, no mere scheme of external alteration could ever induce a real resemblance. Every brick would have to be taken down, and every beam and bolt removed. The problem could not be wrought by the remodelling of an old house: the only mode of solving it would be by the erection of a new one.

MAXILLARY BONE?

(One fourth nat. size, linear.)]

Of the upper maxillary bones of the Asterolepis, I only know that a considerable fragment of one of the pieces, recognized as such by Agassiz, has been found in the neighborhood of Thurso by Mr. Dick, unaccompanied, however, by any evidence respecting its place or function. It exhibits none of the characteristic tubercles of the dermal bones, and no appearance of teeth; but is simply a long bent bone, resembling somewhat less than the half of an ancient bow of steel or horn,—such a bow as that which Ulysses bended in the presence of the suitors. By some of the Russian geologists this bone was at first regarded as a portion of the arm or wing of some gigantic Pterichthys. In the accompanying print (fig. 34) I have borrowed the general outline from that of a specimen of Professor Asmus, of which a cast may be seen in the British Museum; while the shaded portion represents the fragment found by Mr. Dick. The intermaxillary bones, like the dermal plates of the lower jaw, were studded by star-like tubercles, and bristled thickly along their lower edges with the ichthyic teeth, flanked by teeth of the reptilian character. The opercules of the animal consisted, as in the sturgeon, of single plates (fig. 35) of great massiveness and size, thickly tubercled outside, without trace of joint or suture, and marked on their under surface by channelled lines, that radiate, as in the other plates, from the centre of ossification. That space along the nape which intervened between the opercules, was occupied, as in the Dipterus and Diplopterus, by three plates, which covered rather the anterior portion of the body than the posterior portion of the head, and which, in the restoration of Osteolepis, (fig. 13,) appear as the plates, 9, 9, 9. I can say scarce any thing regarding the lateral plates which lay between the intermaxillaries and the cranial buckler, and which exist in the Osteolepis, fig. 13, as the plates 2, 4, 5, 6, and 7; nor do I know how the snout terminated, save that in a very imperfect specimen it exhibits, as in the Diplopterus and Osteolepis, a rounded outline, and was set with teeth.

INNER SURFACE OF OPERCULUM OF ASTEROLEPIS.

(One fifth nat. size, linear.)]

HYOID PLATE.

(One ninth nat. size, linear.)]

That space comprised within the arch of the lower jaws, in which the hyoid bone and branchiostegous rays of the osseous fishes occur, was filled by a single plate of great size and strength, and of singular form, (fig. 36;) and to this plate, existing as a steep ridge running along the centre of the interior surface, and thickening into a massy knob at the anterior termination, that nail-shaped organism, which I have described as one of the most characteristic bones of the Asterolepis, belonged. In the Osteolepis, the space corresponding to that occupied by this hyoid plate was filled, as shown in fig. 14, by five plates of not inelegant form; and the divisions of the arch resembled those of a small Gothic window, in which the single central mullion parts into two branches atop. In the Holoptychius and Glyptolepis there were but two plates; for the central mullion, i. e. line of division, did not branch atop; and in the Asterolepis, where there was no line of division, the strong nail-like bone occupied the place of the central mullion. The hyoidal armature of the latter fish was strongest in the line in which the others were weakest. Each of the five hyoid plates of the Osteolepis, or of the two plates of the Glyptolepis or Holoptychius, had its own centre of ossification; and in the single plate of Asterolepis, the centre of ossification, as shown by the radiations of the fibre, was the nail-head. This head, placed in immediate contact with the strong boxes of bone which composed the under jaw, just where their central joining occurred, seems to have lent them a considerable degree of support, which at such a juncture may have been not unnecessary. In some of the nail-heads, belonging, it is probable, to a different species of Asterolepis from that in which the nail figured in page 7, and the plate in the opposite page, occurred,—for its general form is different, (fig. 37,)—there appear well-marked ligamentary impressions closely resembling that little spongy pit in the head of the human thigh-bone to which what is termed the round ligament is attached. The entire hyoid-plate, viewed on its outer side, resembles in form the hyoid-bone,—or cartilage rather,—of the spotted dog-fish, (Scyllium stellare;) but its area was at least a hundred times more extensive than in the largest Scyllium, and, like all the dermal plates of the Asterolepis, it was thickly fretted by the characteristic tubercles. In the Ray, as in the Sharks, the piece of thin cartilage of which this plate seems the homologue, is a flat, semi-transparent disk; and there is no part of the animal in which the progress of those bony molecules which encrust the internal framework may be more distinctly traced, as if in the act of creeping over what they cover, in slim threads or shooting points,—and much resembling new ice creeping in a frosty evening over the surface of a pool.

NAIL-LIKE BONE OF HYOID PLATE.

(One half nat. size.)]

That suite of shoulder-bones that in the osseous fishes forms the belt or frame on which the opercules rest, and furnishes the base of the pectorals, was represented in the Asterolepis, as in the sturgeon, by a ring of strong osseous plates, which, in one of the two species of which trace is to be found among the rocks of Thurso, were curiously fretted on their external surfaces, and in the other species comparatively smooth. The largest, or coracoidian plate of the ring, as it occurs in the more ornate species, (fig. 38,) might be readily enough mistaken, when seen with only its surface exposed for the ichthyodorulite of some large fish, allied, mayhap, to the Gyracanthus formosus of the Coal Measures; but when detached from the stone, the hollow form and peculiar striæ of the inferior surface serve to establish its true character as a dermal plate. The diagonal furrowings which traversed it, as the twisted flutings traverse a Gothic column moulded after the type of the Apprentice Pillar in Roslin chapel, seem to have underlaid the edge of the opercule; at least I find a similar arrangement in the shoulder-plates of a large species of Diplopterus, which are deeply grooved and furrowed where the opercule rested, as if with the design of keeping up a communication between the branchiæ and the external element, even when the gill-cover was pressed closely down upon them. And,—as in these shoulder-plates of the Diplopterus the furrows yield their place beyond the edge of the opercule to the punctulated enamel common to the outer surface of all the creature’s external plates and scales,—we find them yielding their place, in the shoulder-plates of the Asterolepis, to the starred tubercles.

SHOULDER (i. e. CORACOID?) PLATE OF ASTEROLEPIS.

(One third nat. size, linear.)]

DERMAL BONES OF ASTEROLEPIS.

(One third nat. size, linear.)]

INTERNAL BONES OF ASTEROLEPIS.

(One half nat. size, linear.)]

A few detached bones, that bear on their outer surfaces the dermal markings, must have belonged to that angular-shaped portion of the head which intervened between the cranial buckler and the intermaxillary bone; but the key for assigning to them their proper place is still to find; and I suspect that no amount of skill on the part of the comparative anatomist will ever qualify him to complete the work of restoration without it. I have submitted to the reader the cranial bucklers of five several genera of the ganoids of the Old Red Sandstone; but no amount of study bestowed on these would enable even the most skilful ichthyologist to restore a sixth; nor is the lateral area of the head, which was, I find, variously occupied in each genus, less difficult to restore than the buckler which surmounted it. Two of the more entire of these dermal bones I have figured (fig. 39, a and b) in the hope of assisting future inquirers, who, were they to pick up all the other plates, might yet be unable, lacking the figured ones, to complete the whole. The curiously-shaped plate a, represented in its various sides by the figures 1, 2, 3, is of an acutely angular form in the transverse section, (the external surface, 1, forming an angle which varies from thirty to forty-five degrees with the base, 3;) and as it lay, it is probable when in its original place, immediately under the edge of the cranial buckler, it may have served to commence the line of deflection from the flat top of the head to the steep descent of the sides, just as what are technically termed the spur-stones in a gable-head serve to commence the line of deflection from the vertical outline of the wall to the inclined line of the roof, or as the spring-stones of an arch serve to commence the curve. A few internal bones in my possession are curious, but exceedingly puzzling. The bone a, fig. 40, which resembles a rib, or branchiostegous ray, of one of the ordinary fishes, formed apparently part of that osseous style which in fishes such as the haddock and cod we find attached to the suite of shoulder-bones, and which, according to Cuvier, is the analogue of the coracoidian bone, and, according to Professor Owen, the analogue of the clavicle. Fig. b is a mere fragment, broken at both ends, but exhibiting, in a state of good keeping, lateral expansions, like those of an ancient halbert. Fig. c, 41, which is also a fragment, though a more considerable one, bears in its thicker and straighter edge a groove like that of an ichthyodorulite, which, however, the bone itself in no degree resembles. Fig. d is a flat bone, of a type common in the skeleton of fishes, but which, in mammals, we find exemplified in but the scapulars. It seems, like these, to have furnished the base to which some suite of movable bones was articulated,—in all likelihood that proportion of the carnal bonelets of the pectoral fins which are attached in the osseous fishes to its apparent homologue, the radius. Fig. e, a slim light bone, which narrows and thickens in the centre, and flattens and broadens at each end, was probably a scapula or shoulder-blade,—a bone which in most fishes splices on, as a sailor would say, by squamose jointings, to the coracoidian bone at the one end, and the super-scapular bone at the other. As indicated by its size, it must have belonged to a small individual: it is, however, twice as long, and about six times as bulky, as the scapula of a large cod.

INTERNAL BONES OF ASTEROLEPIS.

(One third nat. size, linear.)]

ISCHIUM OF ASTEROLEPIS.

(One half nat. size, linear.)]

Of the bone represented in fig. 42, I have determined, from a Cromarty specimen, the place and use: it formed the interior base to which one of the ventral fins was attached. In all fishes the bones of the hinder extremities are inadequately represented: in none do we find the pelvic arch complete; and to that nether portion of it which we do find represented, and which Professor Owen regards as the homologue of the os ischium or hip-bone, the homologues of the metatarsal and toe-bones are attached, to the exclusion of the bones of the thigh and leg. In the Abdominales,—fishes such as the salmon and carp,—that have the ventrals placed behind the abdomen, in the position analogous to that in which the hinder legs of the reptiles and mammals occur, the ischiatic bones generally exist as flat triangular plates, with their heads either turned inwards and downwards, as in the herring, or outwards and downwards, as in the pike; whereas in some of the cartilaginous fishes, such as the Rays and Sharks, they exist as an undivided cartilaginous band, stretched transversely from ventral to ventral. And such, with but an upward direction, appears to have been their position in the Asterolepis. They seem to have united at the narrow neck A, over the middle of the lower portion of the abdomen; and to the notches of the flat expansion B,—notches which exactly resemble those of the immensely developed carpal bones of the Ray,—five metatarsal bones were attached, from which the fin expanded. It is interesting to find the number in this ancient representative of the vertebrata restricted to five,—a number greatly exceeded in most of the existing fishes, but which is the true normal number of the vertebrate sub-kingdom as shown in all the higher examples such as man, the quadrumana, and in most of the carnaria. The form of this bone somewhat resembles that of the analogous bone in those fishes, such as the perch and gurnard, cod and haddock, which have their ventrals suspended to the scapular belt; but its position in the Cromarty specimen, and that of the ventrals in the various specimens of the Cœlacanth family in which their place is still shown, forbids the supposition that it was so suspended,—a circumstance in keeping with all the existing geological evidence on the subject, which agrees in indicating, that of the low type of fishes that have, monster-like, their feet attached to their necks, the Old Red Sandstone does not afford a trace. This inferior type, now by far the most prevalent in the ichthyic division of the animal kingdom, does not seem to have been introduced until near the close of the Secondary period, long after the fish had been degraded from its primal place in the fore front of creation. In one of my specimens a few fragments of the rays are preserved, (fig. 43, b.) They are about the eighth part of an inch in diameter: depressed in some cases in the center, as if, over the internal hollow formed by the decay of the cartilaginous centre, the bony crust of which they are composed had given way; and, like the rays of the thornback, they are thickened at the joints, and at the processes by which they were attached to the ischiatic base. It may be proper, I should here state, that of some of the internal bones figured above I have no better evidence that they belonged to the Asterolepis, than that they occur in the same beds with the dermal plates which bear the characteristic star-like markings,—that they are of very considerable size,—and that they formed no part of the known fishes of the formation.

a. Single joint of ray of Thornback.