lOO 



REVISION OF THE PELYCOSAURIA. 



reptilia does not fill the brain cavity, but is supported by a mass of connective tissue 

 carrying lymph and fat masses; so a cast of the brain cavity does not give an exact 

 copy of the brain. However, many points can be brought out by such a cast. 



" If the cast be held with the short terminal portion of the medulla horizontal, 

 the lower surface pitches downward at a sharp angle to a point anterior to the tympanic 

 region, and then ascends as sharply to the point of origin of the hypophysis. The 

 superior surface is horizontal and arched from side to side to a point over the 

 tympanic cavity and there turns upward at an angle of 45°. The angle thus pro- 

 duced is marked by a low, narrow ridge running across the cast and marking the 



position on the brain of a narrow and elevated 

 cerebellum, fig. 27 cb^ such as occurs in Spheno- 

 doii. This region was probably the seat of a large 

 amount of connective tissue, and it is probable that 

 the upper surface of the medulla descended at as 

 sharp an angle as the lower. This would make 

 still more marked the resemblance to Sphenodon 

 and to the cast figured by Cope. This sharp bend 

 of the medulla downward is not found in other 

 forms, though in the brain of Chelonia and some 

 Lacertilia a bend is apparent. 



"The sides of the medulla show most pos- 

 teriorly the beginning of the twelfth nerves, figs. 

 27 and 28 (12), anterior to these the cast of the 

 jugular foramen, y«, and finally the large casts of 

 the tympanic cavity, Ty. 



"Anterior to the tympanic casts a sharp con- 

 striction marks the ridge defining the limits of the 

 tympanic cavity, and then a sharp outswelling, the 

 point of exit of the trigeminus nerve (5). Near 

 where these leave the body of the cast a small 

 stub on each side marks the origin of the seventh 

 pair (7). 



" The hypophysis is the most interesting 

 feature of the brain. Descending between the 

 anterior-inferior process of the petrosal and turning 

 posteriorly, it occupies a small notch in the pos- 

 terior edge of the upper surface of the basisphenoid 

 and then passes directly into the body of the basi- 

 occipital through the foramen mentioned. In the 

 Crocodilia a somewhat similar condition exists." 

 Some additional points have been made out from specimens looi and 1002, 

 University of Chicago. The distal ends of the paroccipitals rest on or close to the 

 upper edges of the quadrates and are overlapped by the squamosals. On the left side 

 of the cranial region of specimen 1002 the median portion of the stapes is preserved ; 

 it shows that tlie stapes was a slender rod extending from the foramen to the quadrate 

 just beneath the paroccipital. Unfortunately neither end is preserved. Cope speaks 

 of both a columella auris and a stapes, but there is no evidence of more than a single 

 bone in these specimens. The semicircular canals of both sides are fairly well pre- 

 served and show the presence of a large ampullar space (ampuUenraum, Siebenrock) 



Fig. 27. — A lateral view of cast of brain cavity 

 of D. inci'sivus, No. 1 University of Chi- 

 cago. X 1. ("6, cerebellum ; 12, tv^elfth 

 cranial nerve ; jii, cast of jugular for- 

 amen. Ty, cast of tympanic cavity ; 

 Hy, hypophysis; 5, 7, fifth and seventh 

 cranial nerves. 



Fig. 28. — Ventral view of same. Lettering as 

 in fig. 27. 



