Page:The Osteology of the Reptiles.pdf/44

26 Basisphenoid (bs). (Figs. 10, 11, 30.) Forming the floor of the brain-case in front of the basioccipital, continuous with the parasphenoid in front (Fig. 12),

which is closely fused with its under side, articulating in front above with the postoptics (Fig. 69 ), behind above with the proötics (Figs. 30, 69 ), externally above with the stapes (Fig. 11 ), and externally below with the pterygoids. It lodges in front the fossa or depression for the pituitary body (Fig. 69 ).

Stapes (st). (Fig. 11 .) The stapes in all early reptiles is a large bone, articulating over the auditory opening, or foramen ovale, between the paroccipital and basisphenoid, and extending toward or touching the quadrate. It is perforated near its proximal end by the foramen for a perforating artery. It is also large in the Ichthyosauria and Plesiosauria, but in most other reptiles is slender, without a perforating foramen. It is stout and short in the Amphisbaenia (Fig. 56).

Parasphenoid (ps). (Figs. 6, 10 21 .) A membranous, unpaired bone, firmly fused in the adult with the under side of the basisphenoid, and never a separate bone. It extends far forward as a narrow rostrum in the temnospondyls (Fig. 11 ) and some cotylosaurs (Figs. 6, 21 ) quite to the prevomers, forming the floor to the ethmoidal cavity. This seems to be the rule in the early reptiles, though in some (Labidosaurus sp) it