The skull is depressed ; its length is rather more than thrice its breadth ; but the proportions somewhat vary in different species. The cranial part, or that behind the orbits, is quadrate ; thence it contracts laterally to near the maxillo premaxillary suture, where it continues either parallel or with a slight swelling before rounding into the obtuse anterior termination.
The orbits are at or near the middle of the skull : estimat ing the length of this by that of the lower jaw, they are in advance of the middle part in Plesiosaurus Hawkinsii. The orbits are rather subtriangular than round, being somewhat squared off behind, straight above, and contracted anteriorly. No trace of sclerotic plates has yet been discerned in any specimen. The temporal fossfe are large subquadrate apertures. The nostrils, which are a little in advance of the orbits, are scarcely larger than the parietal foramen. Beneath them, upon the palate, are two similar-sized apertures, probably the palatal nostrils.
The lower jaw presents an angular, surangular, splenial, and dentary element, in each ramus ; the dentary elements being confluent at the expanded symphysis. There is no vacuity between the angular and surangular or any other element of the jaw. The coronoid process is developed, as in Placodus, from the surangular, but rises only a little higher than in crocodiles. The alveoli are distinct cavities, and there is a groove along their inner border in both jaws.
When the successional teeth first project in that groove, they give the appearance of a double row of teeth. All the teeth are sharp-pointed, long, and slender, circular in cross section, with fine longitudinal ridges on the enamel ; the anterior teeth are the longest.
The scapula is a strong triradiate bone, the longest ray being formed by the acromial or clavicular process, which arches forward and inward to abut against the stermun or epicoracoid.
The proper body of the is short and straight, somewhat flattened ; the thick articular end, which forms the _IL —I_ - 1! • 1 1 ray, is suoequany uiviueu by the articular surface for the coracoid, and that for the head of the humerus.
The coracoids are chiefly re markable for their excessive ex pansion in the direction of the axis of the trunk, extending from the abdominal ribs forward, so as . to receive the entosternum, which is wedged into their anterior inter space. The median borders meet and unite for an extent deter mined by theirdegree of curvature or convexity, which is always slight. The coracoids unite an teriorly with the clavicles, as well as with the episternum ; laterally they articulate with the scapula, combining to form the glenoid cavity for the humerus.
The episternum has the same general form as the median pieces of the abdominal ribs, being, like those pieces, a modified luemal spine, only more advanced in position ; the lateral wings or prolongations are broader and flatter ; the median process is short ; a longitudinal ridge pro jects from the middle of the in ternal surface. The humerus is
a moderately thick and long bone, with a convex head, sub-cylindrical at its proximal end, becoming flattened and gradually expanded to its distal end, where it is divided into two indistinct surfaces for the radius and ulna. The shaft in most species is slightly curved backwards, or the hind border is concave, whilst the front one is straight. The radius and ulna are about half the length of the humerus ; the former is straight, the latter curved or reniform, with the concavity towards the radius ; both are flattened ; the radius is a little contracted towards its carpal end, and in some species is longer than the ulna. The carpus consists of a double row of flat rounded discs,—the largest at the radial side of the wrist ; the ulnar or hinder side appear ing to have contained more unossified matter. The meta carpals, five in number, are elongate, slender, slightly expanded at the two ends, flattened, and sometimes a little bent. The phalanges of the five digits have a similar form, but are smaller, and progressively decrease in size ; the expansion of the two ends, which are truncate, makes the sides or margins concave. The first or radial digit has generally three phalanges, the second from five to seven, the third eight or nine, the fourth eight, the fifth five or six phalanges. All are flattened ; the terminal ones are nailless ; and the whole were obviously included, like the paddle of the porpoise and turtle, in a com mon sheath of integument. The pelvic arch consists of a short but strong and straight narrow moveable. ilium, and of a broad and flat pubis and ischium ; the former subquadrate or subcircular, the latter triangular ; the fore-and-aft expanse of both bones nearly equals that of the corticoids. All concur in the formation of the hip-joint. The ischium and pubis again unite together near their menial borders, leaving a wide elliptic vacuity, or "foramen ovale," between this junction and their outer acetabular one. The pelvic paddle is usually of equal length with the pectoral one, but in P. maerocephalus it is longer. The bones closely correspond, in number, arrange ment, and form, with those of the fore limb. The lemur has the hind margin less concave, and so appears more straight. The fibula, in its reniform shape, agrees with its homotype the ulna. The tarsal bones are also smallest on the tibial side. Of existing reptiles, the lizards, and amongst these the old world Monitors (Varanus, Fitz.), by reason of the cranial vacuities in front of the orbits, most resemble the Plesiosaur in the structure of the skull. The division of the nostrils, the vacuities in the occipital region between the exoccipitals and tympanics, the parietal foramen, the zygomatic extension of the post-frontal, the palato-maxillary, and pterygo-sphenoid vacuities in the bony palate, are all lacertian characters, as contradistinguished from crocodilian ones.