Dec. 15, 1882.] 5bd [Cope. 



Bridger Eocenes. In later epochs it is chiefly seen only in the last supe- 

 rior molar. 



It is also evident that the quadritubercular molar is derived from the 

 tritubercular by the addition of a lobe of the inner part of a ciagulum of 

 the posterior base of the crown. Transitional states are seen in some of 

 the Periptychidce (Anisonchus) and in the sectorials of the Procyonidce. 



On the Brains of the Eocene Mammalia Phenacodus and Periptychus. By 



KB. 



(Bead before the American Philosophical Society, December 15, 1882.') 



PHENACODUS PRIM^EVUS Cope. 



A cast of the cranial cavity gives the following as the general characters 

 of the brain. The cerebal hemispheres are remarkably small, each one 

 being less by one-quarter than the cerebellum. They are separated from 

 the latter and from the large olfactory lobes by strong constrictions. The 

 posterior one is occupied by a thick tentorium. In like manner a wide 

 groove for a robust falx separates the hemispheres above, a notch repre- 

 sents the sylvian fissure, and the lobus hippocampi is quite large. The 

 vermis of the cerebellum is quite distinct, and the lateral lobes are large. 

 They are impressed laterally by the petrous bones as in various ruminants. 

 The anterior columns of the medulla are not visible. There are traces of 

 the convolutions on their hemispheres. 



The brain displays the following more special features. The olfactory 

 lobes are as wide as long, and they diverge, having two external sides. 

 In section they are triangular, presenting an angle downwards. The 

 hemispheres are depressed, and wider posteriorly. They are well sepa- 

 rated from each other and from the cerebellum ; so much so that it is 

 quite probable that the copora quadrigemina are exposed. Their outlines 

 are however not distinguishable on the flat surface which connects the 

 hemispheres posteriorly. No further indication of sylvian fissure can be 

 seen in the cast beyond an entering angle defining the lobus hippocampi 

 anteriorly. The latter is prominent externally, and less so downwards. 

 There are distinct indications of convolutions. There are three on each 

 side above the sylvian convolution, and a fourth extends from the sylvian 

 upwards and posteriorly below the posterior part of the third or external 

 convolution. The sulci separating the convolutions are very shallow. 

 The internal and external convolutions unite anteriorly, passing round the 

 extremity of the median convolution. The space between this gyrus and 

 the base of the olfactory lobe is only three millimeters. 



