Page:The Osteology of the Reptiles.pdf/233



region of skull wholly roofed over, or secondarily emarginated from the sides, not perforated.

Temporal roof complete, not emarginated: skeleton primitive. An order of reptiles, not only the oldest geologically, but more primitive in structure than any other; probably the ancestral stock of all later Amniota. They were very variable in size, structure, and habits: as known, subaquatic, lowland, or marsh reptiles, never cursorial or climbing; invertebrate feeders for the most part, varying in size from less than one to about ten feet in length. The body was never slender, nor the legs long; the neck was always short. Dermal ossifications of any kind are known in but a few genera, Diadectes, Pantylus, and the Pariasauridae; the body was probably covered with horny plates or scales. The earliest known member of the order dates from the Middle Pennsylvanian, the latest from the Middle Triassic.

None has all the primitive characters given in the list, but the losses or modifications in any form are few. They may be discussed under three groups, which include their chief evolutional modifications.

The intertemporal bone and the otic notch are known in but one or two genera, the Seymouriidae; the supratemporals are absent in the Captorhinidae, the tabulars also in Labidosaurus. The ectopterygoid has not yet been certainly demonstrated in any genus, though probably present. The parietal foramen is absent in Pantylus, and possibly also in some others. Free ribs are present to the sacrum and in the tail in Seymouria only, absent wholly in the lumbar region of the Captorhinidae. Parasternal ribs are known only in the last-named family and Sauravus. Two pairs of sacral ribs are present in all genera except Seymouria and Diadectoides. There are from twenty-two or twenty-three to twenty-six presacral vertebrae; the tail of