The pentadactyl limbs are large, and are more or less completely converted into paddles by a flattening and shorten ing of the radius and ulna and tibia and fibula, and an increase in the number of phalanges. The shoulder girdle consists of scapulae and coracoids, which meet one another in median suture. Clavicles are probably always present, an interclavicle usually so. The ilium is small, the pubis and ischium, though separated by an obturator foramen, are expanded into flat sheets of bone. A strong plastron of abdominal ribs is always present.
Sub-order I. Trachelosauria. A single, small reptile, with a long neck consisting of 20 vertebrae whose centra support two headed ribs. Dorsal ribs single-headed and articulating with the long transverse processes of the dorsal neural arches.
Ilium and femur like those of a land reptile.
Lower Trias.
Sub-Order 2. Nothosauria. Sauropterygia in which the limbs are still incompletely converted into paddles, the elbow and knee joints still being flexible. Phalangean formula 3, 4, 5, 3 or 4 In the skull the opisthotic is enlarged distally, and articulates with the squamosal quadrate and pterygoid, so as to close the middle ear cavity behind. Clavicular arch powerful; coracoids A. From the right side, with lower jaw, B. from above, C. from below, D. occiput meeting in a short symphysis, which lies behind a line joining the glenoid cavities. Ilium articulating with both pubis and ischium.
Middle Trias, perhaps just appearing in the Lower Trias. Families not discriminated.
Sub-Order 3. Plesiosauria. Sauropterygia in which the limbs are completely converted into paddles, with no freedom of move ment at any joint ; the number of phalanges in the five fingers and toes is increased, reaching 6, 13, 15, 13, 9 or more. The distal end of the opisthotic is slender, resting on the hinder surface of the squamosal. Clavicular arch, when present, reduced to flat sheets of bone, supported by the greatly enlarged acromia of the scapulae. Coracoids with a symphysis which extends forward between the glenoid cavities. Ilium articulating only with the ischium. Rhaetic to Upper Cretaceous. Families not yet dis criminated.
Sub-order 4. Placodontia. Sauropterygia in which the skull has become modified to support great crushing teeth in the maxil lae and palatines. Neck with eight vertebrae, the cervical ribs articulating with both centrum and neural arch ; dorsal vertebrae with concave articulations, ribs attached solely to the neural arch.
Limb girdles essentially like those of Nothosauria, fore limb some what paddle-like, but with the primitive number of phalanges, Femur like that of a land animal. A well-developed armour of dermal ossifications both dorsally and ventrally. Middle Trias to Rhaetic.
Order Ichthyosauria. Reptiles which are fully adapted for a marine life. The head is elongated, the neck short and the tail very long and powerful, provided with a terminal fin, which is the most important swimming organ.
The limbs are paddles, they are never large, and the hind limbs may become very small. The skull has a single temporal fossa which is surrounded by the parietal, supratemporal (often called squamosal) postfrontal, and sometimes frontal. This opening thus appears to differ in its boundaries from all found in other reptiles. The deep, but usually very short temporal arcade is very largely formed by a bone, often called the supratemporal, which is, perhaps, the true squamosal ; it contains also processes of the postfrontal and supratemporal, and a quadrato-jugal. The postorbital and jugal are narrow bones round the enormous orbit.
The nostril lies immediately in front of the orbit, and the long rostrum is built up from the premaxillae and nasals. The bicon cave vertebral centra are extremely short, and the neural arches feeble. The ribs are two-headed at least anteriorly, and articulate solely with the centra. There is no sacrum. The hinder part of the tail is downturned very slightly in Triassic forms, nearly at right angles in those from the Upper Jurassic, in order that it may support the lower lobe of a vertical caudal fin whose upper lobe has an unossified skeleton.
The shoulder girdle consists of scapulae and corticoids which meet in a powerful median symphysis between the glenoid cavi ties, and a rigid clavicular arch. The pelvis has a narrow ilium and pubis and ischia, separated by an obturator foramen, but expanded into great sheets in Triassic forms. In both fore and hind limbs the proxjmal bone is very short and widened distally, the remainder of the limb, in the later forms, being reduced to an interlocking mass of polygonal bones. The number of fingers is often increased to seven or more, and the phalanges increase to a very great number.