26 



THE OSTEOLOGY OF THE REPTILES 



Fig. 12. Parity lus. Cotylosaur skull 

 and mandible: immature animal, 

 from below. Natural size. 



Basisphenoid (bs). (Figs. lo, ii d, 30.) Forming the floor of the 



brain-case in front of the basioccipital, continuous with the para- 

 sphenoid in front (Fig. 12), which is 

 closely fused with its under side, ar- 

 ticulating in front above with the 

 postoptics (Fig. 69 d), behind above 

 with the prootics (Figs. 30, 690), 

 externally above with the stapes 

 (Fig. II c), and externally below 

 with the pterygoids. It lodges in 

 front the fossa or depression for the 

 pituitary body (Fig. 69 d). 



Stapes (st). (Fig. 11 c, d.) 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). 



Paras phenoid (ps). (Figs. 6, 10 c, d, 

 21 A.) 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. 1 1 e-g) and some 

 cotylosaurs (Figs. 6, 21 a) 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 



' [It is separate in at least some geckos. — G. K. N.] 



Fig. 13. Parity lus. Cotylosaur skull: 

 internal cranial bones. Natural size. 

 A, basioccipital; B, basisphenoid; C, 

 section at front of basisphenoid; D, 

 section of rhinencephalic chamber op- 

 posite hind margin of orbits; E, out- 

 line of same at extreme front end of 

 parietals; F, same in front of orbits; 

 pal, prevomer. 



