OF THK PELYCOSAURIAN GENUS DIMETRODON. 



13 



ninth or tenth nerves or both of them. In Sphenodon the foramen transmits not only 

 these but the twelfth pair as well, the nerves being separated from the vein by very 

 thin walls of bone and may be separated from each other or have a common canal. 

 The opening of the twelfth pair into the notch which forms the beginning of the 

 jugular foramen is then very similar to the condition found in Sphenodon. 



Fig. 1. Lateral view of the cast of the brain cavity 

 of the Dimetrodon incisivus, specimen No. 1. Cb., cere- 

 bellum ; TV., cast of the otic cavity ; Hy., hypophysis ; 

 Ju., cast of jugular foramen. 5, 7, 12, casts of the fora- 

 mina tor the corresponding cranial nerves. 



Fio. 2. Inferior view of the same cast. Lettering 

 as in Fig. 1. 



"The fenestra ovalis is a single opening leading by a very short canal directly into 

 the brain cavity, a character found in fishes and the amphibian Menopoma and existing 

 imperfectly in some recent reptilia, as the turtles. The same thing is described by 

 Cope as existing in another Permian reptile, from the same horizon as the present 

 specimen, but belonging to a separate family, the Diadeciidie, and his order Cotylosauria. 



" The foramina for the seventh (facial) pair of nerves appear on the outer surface 

 of the petrosal just anterior to the fenestra ovalis. They are located relatively a little 

 further back than in Sphenodon. On the inner face of the same bone the foramina 

 appear at the side of the base of the brain cavity a little anterior to their external 

 opening. They are located just anterior to a slight ridge which defines the limits of 

 the tympanic cavity. In Sphenodon this is about the point of location of a foramen 

 common to the seventh and eighth nerves, which, however, almost immediately divides, 

 the posterior branch penetrating the inner wall of the tympanic cavity and leading 

 the auditory nerve to the inner ear. 



"The foramen for the fifth (trigeminus) nerve is completed from the incisura 

 otosphenoidea by the membranous wall of the anterior portion of the brain case, as in 

 Sphenodon and many lizards. 



A. p. S.— XXL B. 



