A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 43 



opening into the mouth. In the present form the tubes probably penetrated the large 

 mass of cartilage covering the otic region and the posterior end of the basisphenoid and 

 found a common opening in the deep pit described. It is difficult to imagine the use of 

 such an extensive cavity in the basisphenoid, but in the Teleosauria an equally large 

 cavity is found roofed over with bone. Anterior to this pit two foramina, Fig. 5, /. C, 

 penetrate the lower surface of the basisphenoid bone and on its upper surface a large 

 foramen appears just posterior to the origin of the presphenoid rostrum. Through the 

 pair on the lower surface the internal carotid arteries enter the bone and through the 

 upper it gains access to the brain cavity by way of the pituatary fossa. On either side 

 of the single foramen a pair of small foramina carry branches of the internal carotid. 

 All of these foramina are very similar in position to the same ones in Sphenodon. 



The cast of the brain cavity shows fairly well all parts posterior to the fifth pair 

 of nerves, and the hypophysis anterior to them. As is well known, the brain in the 

 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 downwards 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 supe- 

 rior surface is horizontal and arched from side to side to a point over the tympanic cavity 

 and there turns upwards at an angle of 45°. The angle thus produced is marked by a 

 low, narrow ridge running across the cast and marking the position on the brain of a 

 narrow and elevated cerebellum, Figs. 4 and 7, Cb., such as occurs in Sphenodon. 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 downwards 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 posteriorly the beginning of the twelfth nerves, 

 Figs. 4 and 7 (12), anterior to these the cast of the jugular foramen, Figs. 4 and 7, Ju., 

 and finally the large casts of the tympanic cavity, Figs. 4 and 7, Ty. The nature of 

 the matrix and the cavities prevented the tympanic cavities being cleaned so that the 

 semicircular canals could be determined, but it is probable that they were very similar 

 to those described by Cope. 



Anterior to the tympanic casts a sharp constriction marks the ridge defining the 

 limits of the tympanic cavity and then a sharp outswelling the point of exit of the tri- 

 geminus nerve, Figs. 4 and 7 (5). Near where these leave the body of the cast a small 

 stub on each side marks the origin of the seventh pair, Figs. 4 and 7 (7). 



