The limbs are of the terrestrial type; the five digits have the usual number of phalanges, which do not exceed five. The bones of the limbs are slender; the humerus has an entepicondylar foramen.
Fam. 1. Mesosauridae.–The neck contains about ten vertebrae. The vertebrae are deeply biconcave, perforated by the chorda dorsalis. Sacral vertebrae four in number. Clavicles strong; interclavicle very small. Mesosaurus, the only genus, with one species, M. tenuidens, about one foot in length, was found in South Africa, probably in Triassic sandstone. Very similar specimens are known from São Paolo in Brazil.
Fam. 2. Nothosauridae.–With sixteen to twenty-one cervical and three to five sacral vertebrae. The vertebrae are biconcave. The clavicles are strong; the interclavicle is much reduced. Coracoids with distinct acromial processes.
Nothosaurus mirabilis, of the Muschelkalk of Germany. Total length about ten feet. Length of head about one foot. The teeth are very irregular. About five slender, long teeth are implanted in each side of the premaxilla, with wide spaces between them, similar to those of the symphysial portion of the lower jaw. Those of the maxillaries are numerous and small, except two large pairs in the anterior portion, on a level between the orbits and nostrils. The upper and lower teeth overlap, or cross each other. The palate of the long and slender skull is quite bony, without anterior palatal or infra-orbital vacuities.
Lariosaurus balsami, about one foot in length, from the fresh-water deposits of the Upper Trias in Lombardy. Neck with about twenty, tail with about forty vertebrae. Head comparatively shorter; more triangular than in Nothosaurus; palate with small infra-orbital vacuities. The number of the phalanges of the fingers and toes is apparently 2, 3, 4, 4, 3 and 2, 3, 4, 5, 4.
Anarosaurus pumilio, of the Muschelkalk, near Magdeburg, and Neusticosaurus and Simosaurus of the same geological age, are allied forms.
Order II. PLESIOSAURI.
The limbs are transformed into hyperphalangeal paddles. The clavicles are small, and are overlapped ventrally by the strongly developed acromial processes of the scapulae. The vertebrae are slightly biconcave or plane. The neck consists of at least twenty vertebrae; those of the thoracic region have long transverse processes; the sacral vertebrae are mostly reduced to two or one. Very large, massive animals.
Fam. 1. Pliosauridae.–About twenty cervical vertebrae, with proximally bifurcated ribs. The scapulae do not meet ventrally; they enclose with the coracoids a single large foramen, and are fused with the clavicles. Pliosaurus, the principal genus, contains several species of gigantic size; for instance, P. grandis, of the Kimmeridge clay, Upper Oolite, of England, has a skull nearly 5 feet long and 2 feet broad, armed with many enormous conical teeth, some of which reach one foot in length, inclusive of the long collar and root-portion. The neck is rather short, owing to the much condensed, disc-shaped centra of the vertebrae. Total length of this species about 30 feet. Other species in England and continental Europe as far as Russia.
Fam. 2. Plesiosauridae.–The neck is very long, and consists of from twenty-eight to forty vertebrae. The scapulae do not meet ventrally, but the symphysial portion of the coracoids meets the clavicles and the interclavicle, the pectoral arch thus enclosing two foramina. Chief genus Plesiosaurus, with many species. The head is comparatively small, the neck very long, the tail short, although consisting of from thirty to forty vertebrae. The third digit (Fig. 113, C) is the longest, and possesses nine or ten phalanges. The abdominal ribs are very strong, and reach from the pectoral to the pelvic girdle. Range from the Lower Trias to the Lower Oolite, chiefly European. P. dolichodirus and P. conybeari, the latter reaching a total length of more than 15 feet, from the Lower Lias, especially at Lyme Regis.