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  Section: Anatomy of Vertebrate Animals » The Classification and the Osteology of the Reptilia
 
 
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The Ichthtosauria

 
     
 

a restoration of Ichthyosaurus. The existence of the caudal fin is doubtful
Fig. 75. - a restoration of Ichthyosaurus. The existence of the caudal fin is doubtful
In its general form Ichthyosaurus presents a good deal of resemblance to a Cetacean. The head is enormous, and passes at once into the trunk, so that there is no more appearance of a neck than in a Porpoise, and the body tapers off behind, much as would happen in the latter animal were it devoid of a candal fin. Indeed, there is some reason to suspect that the tail of Ichthyosaurus may have been provided with a sort of fin-like expansion of the integument. This fish-like body was propelled, like that of the Plesiosaurus, by four paddles; but the anterior paddles were placed close behind the head, and were, generally, very much larger than the posterior ones.

The spinal column is only distinguished into two regions, caudal and precaudal, in as much as the ribs, beginning at the anterior part of the neck, are continued, without being connected with the sternum, to the posterior end of the body; and there is no sacrum. The caudal region, however, is distinguished by the chevron-bones which are attached beneath its vertebrae. The vertebrae of Ichthyosauria in general have certain characters by which they differ from those of all other Vertebrata. (Fig. 76, C.) Not only are the centra flattened disks, very much broader and higher than they are long, and deeply biconcave (circumstances in which they resemble the vertebrae of some Labyrinthodonts and Fishes), but the only transverse processes they possess are tubercules, developed from the sides of these centra; and the neural arches are connected with two flat surfaces, one on each side of the middle line of the upper surface of the vertebrae, by mere synchondroses. The neural arches themselves are forked bones, with only rudiments of zygapophyses, and in the greater part of the body do not become articulated with one another at all.


Fig. 76. - Different parts of the skeleton of Ichthyosaurus intermedius drawn to the sama scale. A, the skull; B, the fore-limb: II, humerus; R, radius; U, ulna;
r. i. u., radiale, intermedium, ulnare; Cp, carpalia; 1, 2, 3, 4, 5, digits: m. r. m. u.. radial and ulnar marginal ossicles.-C, a dorsal vertebra, with the ribs (R) and ventral ossifications (V.O).-D. the hind-limb: F, femur; T, tibia; Fb, fibula; t, i, f, tibiale, intermedium, fibulare; Ts. tarsalia; Mt, metatarsalia; Ph, phalanges; m, tb, tibiale marginal ossicles -E, the pectoral arch, seen from the ventral side; F, the same aspect of the pehic arch.
In the cervical region, if one may call "neck" the most anterior part of the vertebral column, the front part of the lateral surface of each vertebra presents two separate elevations, or articular surfaces, which are at first situated in the upper half of the lateral surface. Toward the posterior half of the dorsal region they descend, and, gradually approaching one another, coalesce into one in the caudal vertebrae. The form of the proximal ends of the ribs corresponds with the arrangement of these tubercles; for, where they are separate, the proximal end of the rib is forked. The lower fork, or capitulum, goes to the capitular, or lower, tubercle, and the upper branch, or tuberculum, to the upper, or tubercular, elevation. In the caudal region, where the articular surface is single, the proximal end of the rib is also undivided. In the caudal region the ribs are short and straight, but in the precaudal region they are stout and curved, and much longer in the middle than at either end of the series. The atlas and axis resemble the other vertebrae in their general form: but a wedge-shaped bone is, as it were, let in between their opposed lower edges; and a similar bone, attached to the under-part of the concave face of the centrum of the atlas, serves to complete the cup for the occipital condyle.

The skull of Ichthyosaurus (Fig. 76, A) is remarkable for the great elongation and tapering form of the snout, the huge orbits, the great supra-temporal fossae, and the closing over of the infra-temporal fossae by plates of bone. Again, the two rami of the mandible are united in a symphysis, which, for length, is comparable to that observed in the modern Gaviala and in the ancient Teleosauria. The basi-occipital bone furnishes the round articular condyle to the first vertebra, and becomes very stout and thick in front. It appears to have been anchylosed neither with the basisphenoid nor with the basi-occipital. The latter bones are adapted to its sides, and, together with the supra-occipital, which is interposed between them above, circumscribe the occipital foramen. The basisphenoid, a deep and stout bone, is produced in front into a long and slender parasphenoidal rostrum. There do not appear to have been any ossified alisphenoids. The parietals remain separate throughout life; and, in some species, not merely present a great parietal foramen close to the coronal suture, but are completely divided by a median fissure. Ossified presphenoids and orbitosphenoids appear to have been altogether absent, and the frontal bones are relatively small. The prootic bones are, as usual, situated in front of the ex-oocipitals, and between the latter and them there may sometimes be discerned a conical bone with a broad base, which appears to be fitted in between the ex-occipital and the prootic. If this bone were not so large, it might well be regarded as a stapes, but it is possible that, as Cuvier suggests, it answers to the separate opisthotic of the Chelonia.

In the naso-premaxillary segment, the nasal bones, continuing the direction of the frontals, attain considerable size, but the premaxillae make up by far the greater part of the snout. The maxillae are reduced, as in birds, to comparatively small and slender rod-like bones, bounding only a fraction of the gape. The vomers are elongated, and situated in the middle line on the underside of the snout.

The nostrils are small apertures close to the orbits, bounded by the nasal, lachrymal, and premaxilllary bones.

On each side of the frontal there is a large prefrontal, which passes back above to meet the post-frontal, and thus bound the orbit. Below, the maxilla is connected with a jugal. From the post-frontal to the jugal, the posterior margin of the orbit is constituted by a distinct, curved, postorbital bone (Fig. 76, A, Pt. O). A broad and flat quadrato-jugal (Q.j) passes from the end of the jugal to the lower end of the quadrate, and covers in the lower and posterior part of the infra-temporal fossa. The space between this bone, the postorbital, the post-frontal and the squamosal, is occupied by another flattened bone (Fig. 76, A, St.), which Cuvier calls the temporal, but which does not appear to have any precise homologue among other Reptilia. The squamosal bone is very large and stout, and forms the postero-external angle of the skull. From this point it sends a process forward to meet the post-frontal, inward to unite with the parietal, and downward to become connected with the pterygoid. A strong and stout quadrate bone is connected with the exterior of the skull, and presents a pulley-like surface to the articular piece of the mandible.

On the under-surface of the skull the long and slender palatine bones are seen, bounding the posterior nares, which are situated far forward. Behind, and separated by an interval traversed by the rostrum of the basisphenoid bone, the very large pterygoids commence, by slender and pointed ends, which lie on the inner side of the palatine bones at the level of the posterior nares. They then widen, and passing backward with a slight outward curvature, on each side of the sphenoidal rostrum, end in three processes-one which connects itself with the basisphenoid, another passes outward and backward to the quadrate, while the third runs upward to the squamosal bone.

The lower jaw is composed of two rami, which unite, anteriorly, in a very long symphysis. Each ramus is composed of the normal six pieces, the splenial being remarkably long, and entering extensively into the symphysis.

We have no very clear knowledge of the structure of the hyoidean apparatus in this reptile.

The pectoral arch (Fig. 76, E) consists, upon each side, of a narrow scapula (Sc), having the direction usual in Lacertilia, and a broad coracoid (Co.), the inner edge of which does not overlap its fellow, but meets it throughout in the middle line, as in Plesiosaurus: so that, in this genus also, the rhomboidal part of the sternum appears to have been absent or very small.

But there is a very distinct T-shaped interclavicle (I. Cl.), the backward prolongation of which is received between the anterior ends of the coracoids, while its horizontal bar is very closely united with the inner ends of two stout curved clavicles (Cl), the outer extremities of which abut against, and are no less closely connected with, the upper part of the anterior edge of each scapula. This arrangement of the clavicles and interclavicle presents interesting conditions intermediate between those observed in Nothosaurus, on the one hand, and those common in the Lacertilia, on the other.

The scapula and coracoid give rise by their junction to a glenoidal cavity, into which the thick head of the very short prismatic humerus (Fig. 76, B, H) is received. The distal end of the humerus presents two facets, which articulate with a couple of short flattened polygonal bones, which represent the radius and the ulna (R, U). To these succeed two rows of smaller polygonal ossicles in the place of a carpus: three, representing the radiale, intermedium, and ulnure (r. i. u.), lie in the proximal row, and three or four carpalia (Cp.) in the distal row. With the distal carpal bones are connected, by means of the metacarpal ossicles (Mc.) longitudinal series of very numerous polygonal bones, adapted together by their edges, and becoming gradually smaller toward the distal extremity of each series. The number of complete series does not exceed five, and may be reduced to three-so that the paddle may be pentadactyle, tetradactyle, or tridactyle. An apparent multiplication of the number of digits arises from two causes: First, the occasional bifurcation of some of the digits; secondly, the superaddition of marginal bones (I leave open the question whether these series of marginal ossicles are remains of the digits of a polydactyle manus, such as exists in the Elasmobranch fishes.) to the radial and to the ulnar edges of the manus (m. u., m. r.). There is thus formed a paddle, which is unlike either that of a Cetacean, or that of a Plesiosaurus, or that of a Turtle-departing more than any of these structures from the ordinary form of vertebrate limb.

There is no trace of any sternum behind the pectoral arch, but the abdominal walls were strengthened by a number of transverse arcuated bones, similar to those observed in the Plesiosauria, though not so strong. Each is composed of a median piece with pointed ends, and of some three, or more, lateral pieces, overlapping each other's ends, on each side. (Fig. 76, C, V.O.)

The pelvis (Fig. 76, F) is not connected by bone with the vertebral column. It consists of an ilium (Il), an ischium (Is.), and a pubis (Pb.), uniting together to form an acetabulum, while the pubis and ischium of each side meet in the middle line. The ischium is a narrow and almost rod like bone, the pubis is somewhat broader, especially at its symphysial end.

The hind-limb (Fig. 76, D) has substantially the same structure as the fore-limb, but is always smaller, and generally of much less size.

The only other bony structure appertaining to Ichthyosaurus that need be noticed, is a circle of plates developed in the sclerotic of the enormous eye, which is frequently met with in a very perfect state of preservation.

It is possible that the Ichthyosauria occur in the Trias; they abound in the Lias and in other rocks of Mesozoic date, up to, and including, the Chalk.

Some attain gigantic dimensions, and many species have been founded by the differences in form and proportion of the body and of the teeth; but no one form is sufficiently different from the rest to justify its separation as a distinct genus. They may be roughly grouped into such as have relatively short snouts and short paddles, with four carpalia (I. intermedius, communis, etc.); and such as have longer snouts, long paddles, and three carpalia (I. longirostris, tenuirostris, platyodon).


 
     
 
 
     



     
 
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