268 



THE PINEAL ORGAN 



In some extinct reptiles the foramen is farther forward, as in the 

 Ichthyosauri, being situated at the junction of the two frontal bones with 

 the two parietal bones (Fig. 190) ; or it may be far back near the 

 exoccipitals as in Nothosaurus mirabilis. 



In cases in which the lateral or supratemporal fossae are large, these 

 may be separated by a median antero-posterior bony ridge. This is formed 

 chiefly by the inner borders of the parietal bones, but the frontals may 

 participate in its formation, as in Mesosuchus and Mixosaurus atavus 

 (Fig. 191). In some types this ridge is reduced to a relatively slender 

 sagittal crest, as in Trachyodon mirabilis and Iguanodon. In these cases 

 the parietal bones become fused and the interparietal suture and the 



r St.f 



pa r 



'scl.pl 



Fig. 190. — Ichthyosaurus acutirostris, Owen. Upper Lias. Curcy, Calvados. 

 A — Dorsal view of skull. B — Lateral view of skull. 



na. : nasal bone. p. mx. : premaxilla. 



nar. : external nares. scl. pi. : sclerotic plates. 



orb. : orbit. st. f. : supratemporal fossa. 



pa.f. : parietal foramen. 



(After E. Deslongchamps, from Zittel.) 



parietal foramen disappear. These animals were of enormous size ; 

 they walked on their hind legs, using the tail also as a support for the 

 body. Thus the fore-limbs were free and could be used as arms, as in 

 the kangaroos and wallabies. Their temporal muscles were strongly 

 developed, and encroaching on the sides of the skull they obtained an 

 additional origin from the lateral aspects of the crest, which was formed 

 between them, as is the case in the gorilla. A long median crest is also 

 conspicuously developed in certain Ophidia (Python) and Chelonia 

 (Trionyx gangeticus), in neither of which a parietal foramen is present. 

 The formation of the crest will not only tend to reduce the width of the 

 parietal canal, but it will increase the thickness of the skull in this situation 

 and the distance of the cranial cavity from the superficial surface of the 

 skull. Thus, although not the prime cause of the atrophy of the parietal 

 organ and of its vascular and nerve-supply, the evolution of the crest 



