A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 41 



reflected backwards, forming small facets looking up and out. Between the pterygoid 

 processes a slender presphenoid rostrum rises and projects far forwards. This rostrum is 

 thin laterally, expanded vertically, smooth and straight on the lower edge, roughened 

 above. 



The foramina perforating the cranial region and the hrain. 



The foramina penetrating the bones of the cranium in Dimetrodon are remarkably 

 similar in position to those penetrating the same bones in Sphenodon. The condylar 

 foramen transmitting the twelfth pair (hypoglossus) penetrates the exoccipital just ante- 

 rior to the edge of foramen magnum. Its outer end opens in a notch (the incisura venos 

 jugularis Sieb.) in the side of the exoccipital. A little below and further forwards a 

 second and much smaller foramen opens in the same notch ; this may transmit either the 

 ninth or tenth pair of nerves or a minor blood vessel. Passing forwards the notch 

 deepens and is very soon converted into a foramen by the adjacent portion of the paroc- 

 cipital. This is the foramen venos jugularis of Siebenrock, 68 and transmits the jugular 

 vein and either the 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 begin- 

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



The fenestra ovalis, Fig. 6, F. 0., 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 Diadectidce, and his order 

 Cotylosauria. 69 



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

 the petrosal just anterior to the fenestra ovalis (Fig. 6, 7). 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 open- 

 ing. 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 pos- 

 terior 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 oto- 



A. P. S. VOL. XX. F. 



