Fam. 3. Elasmosauridae.–The neck is extremely long, possessing from thirty-five to seventy-two vertebrae, with single-headed, not bifurcated, ribs. The scapulae meet ventrally, and enclose with the very broad coracoids two foramina. The tail is short. The pisiform bone articulates with the humerus. Otherwise much resembling the Plesiosauridae. Principal genus Cimoliasaurus, with many synonyms, and many species from the Middle Oolite to the Upper Chalk; cosmopolitan distribution, e.g. C. cantabrigiensis, of the Greensand and Upper Chalk; C. trochantericus, of the Kimmeridge clay; C. haasti in New Zealand; C. australis, C. chilensis; others in North America. Cryptoclidus of the Middle and Upper Oolite of Europe. Elasmosaurus, of the Upper Cretaceous formation in Kansas, with a computed total length of 45 feet, of which 22 belong to the neck, with its seventy-two vertebrae.
Sub-Class VIII.–ICHTHYOSAURIA.
Marine, whale-shaped reptiles, with the anterior and posterior limbs transformed into hyperphalangeal paddles. Restricted to the Mesozoic age from the Trias to the Upper Chalk.
The skull is long, owing to the elongated slender snout, which is formed mainly by the premaxillary bones. The nostrils lie far back, in front of the orbits, and are bordered by the long nasals, the premaxillaries, a small part of the maxillaries, and posteriorly by the large lacrymal bones. The eyes are large, and are strengthened by a sclerotic ring composed of many closely overlapping bones. The orbits are very large, and are directed sideways so as to be scarcely visible from above. They are formed above by the long prefrontals, which join the postfrontals; behind by the long postorbitals; below by the long and slender jugals; in front by the lacrymals and prefrontals. The postorbito-temporal region of the skull is short but high, and, with the exception of the supratemporal foramen, is entirely closed in by bones, namely, the quadrato-jugals, supratemporals, and squamosals. The latter, with the parietals and large postfrontals, surround the supratemporal foramina. The parietals and the small frontals enclose the parietal foramen. The whole temporal arch consequently recalls much that of the Pareiasauri and Stegocephali, chiefly owing to the presence of conspicuous supratemporal and postorbital bones, which, together with the quadrato-jugal, close in the whole side without any indication of a lateral or infratemporal foramen. The postorbital completely separates the jugal from the quadrato-jugal, and this almost hides the quadrate. The occipital condyle is single. The lateral occipitals and the supra-occipital bones retain their sutures. The pro-otic and opisthotic bones remain separate. The latter lie between the basi- and lateral occipitals, the squamosal, quadrate, and pterygoid. The pterygoids, which posteriorly touch the quadrato-jugals, basi-occipitals, opisthotics, and basisphenoid, are very long and remain widely separated from each other; in the space between them appears the long ensiform presphenoid. Anteriorly they are connected through the ectopterygoids with the maxillae, and touch the palatines. These are likewise narrow and slender, but touch each other in the middle line, and contain the well-separated, slit-like choanae, laterally to which lie the elongated, rather narrow, palatal vacuities. The vomers are mostly not visible; when they appear on the surface they are long and narrow, and enclose the choanae between them and the palatines.
The teeth are pointed, conical and thickly covered with enamel, which in transverse sections forms vertical ridges, recalling the folds of the Labyrinthodonts. The teeth have open roots, and are not implanted in separate alveoli, but lie in long grooves of the premaxillaries, maxillaries, and dentals.
The vertebrae are numerous, up to 150, two-thirds of which belong to the tail. The centra are deeply biconcave and short, not co-ossified with the neural arches, which have therefore often broken loose. The atlas much resembles the other cervical vertebrae in so far as its centrum is concave in front and scarcely ankylosed with that of the second. Its basiventrals, equivalent to the ventral half of the atlas-ring of other reptiles, thus become an unpaired intercentral wedge, between the first centrum and the basis of the cranium; the neural arches rest upon the centrum, but remain separate from each other, or at least diverge dorsally. The atlas carries no ribs. Intercentra occur also between the second and third vertebrae; they reappear in the tail as chevron-bones. All the other vertebrae carry ribs, which gradually increase in length towards the trunk and decrease again equally gradually on the tail. In the neck and trunk they have separate capitula and tubercula, which articulate upon short knobs of the centra; towards the tail these shift farther and farther towards the ventral side, and ultimately unite. Although the ribs of the trunk are so long, there is no trace of a sternum, but there are many "abdominal ribs" crowded together, each consisting of a middle and a pair of lateral pieces.
The shoulder-girdle is very complete, but the pieces remain separate, or at least do not co-ossify; it consists of a T-shaped interclavicle, clavicles, broad coracoids touching each other in the middle line, and short scapulae. The existence of small separate precoracoids is doubtful. The pelvis is much reduced; the small ilium is quite unconnected with any vertebrae; the small pubes and ischia form no symphyses. The fore- and hind-limbs are very similar to each other; the posterior are, however, much smaller. Both are transformed into highly specialised paddles. It is of the greatest importance, as an indication that the Ichthyosauri are descendants of a terrestrial stock, and have been modified into what they are owing to having taken to marine life, that in the oldest members known, the paddle-like structure of the limbs was less advanced than in the later species. In Mixosaurus of the Muschelkalk of Europe the ulna and radius are still distinctly longer than broad, and they enclose a space between them. They articulate with three carpal bones, the ulnare, intermedium, and radiale, while a small pisiform bone lies on the outer side, between the ulnare and the outer distal carpal bone. In Ichthyosaurus, from the Liassic period onwards, the ulna and radius are much shortened, broader than long, and touch each other without any intervening space; the pisiform element is enlarged. Lastly, in Ophthalmosaurus of the Middle Oolite (but not in contemporary species of Ichthyosaurus) the ulna and radius are still more reduced, and the pisiform has moved up to the humerus, so that the latter articulates with three bones.
Fig. 114.–A, Ventral view of the shoulder-girdle and right fore-limb of an Ichthyosaurus, from the Lias; B, part of the fore-limb of a Mixosaurus, from the Trias; C, part of the fore-limb of an Ophthalmosaurus, from the Chalk. c1, c2, first and second centrale carpi; Cl, clavicle; Co, coracoid; H, humerus; I, interclavicle; i, intermedium carpi; p, pisiform; R, radius; r, radial carpal; Sc, scapula; U, ulna; u, ulnar carpal.
Other important features of these paddles are not only the much-increased number of phalanges (sometimes up to twenty or more), but also the increase of digits to six or more, produced apparently by a splitting of the third finger into two series, and by the development of additional rows of phalanx-like bones on the outer and inner margins of the paddle. This increase of fingers exists, for instance, in Ichthyosaurus communis, but not in I. tenuirostris. Owing to this peculiar development of paddles the constituent bones are extremely numerous, and from the radius and ulna downwards they are all closely packed, and have assumed a polygonal, often hexagonal, shape, dwindling to more or less flattened nodules towards the ends of the digits. These carpal and phalangeal bones are common objects in amateurs' collections; they fit together by the short angular facets, while the two flat and broader surfaces are those of the dorsal and ventral sides.