948 EVOLUTIONAL MODIFICATIONS 



iiifj; characteristic in the primate brain. In all species, with the e\cepti(jn of 

 the lemur, often nientionecl as a transitional form, the occipital lobe of the 

 hemisphere has so advanced in its dexelopment as entirel\ to conceal the 

 tentorial surface ol the cerebellum. Not alone in the relation ol the cere- 

 bellar to the cerebral hemispheres are the primates distinguished from all 

 other mammals. There appears another identifying feature on the occipital 

 surface of the cerebellum, i.e. the vallecula, or vermal notch, w hich is most 

 characteristic of the order. The depression between the two lateral cerebellar 

 lobes, w hich lodges the inferior vermis, is also a distinctive mark. The \entral 

 surface of the bram likewise reveals prommeiit mdices connected with the 

 de\elopment of neokinetic functions, such as the ])yramid, the inferior oli- 

 \ary body, the pons Varolii and the cerebral peduncles. 



The De\'elopment of the Pineal Fossa in the Primate Brain. 

 The development of the pineal fossa lying at the cephalic e\tremit\ of tlie 

 intercollicular sulcus makes ]:)ro\ision lor the epiphysis cereljri. The rela- 

 tion of this gland to the midbrain is subject to considerable variation among 

 mammals. In some it occupies a supraeallosal position. In other forms it 

 IS j)ostcaIIosal, lying bi'hind the splenium ol the corpus callosum l)ut not in 

 contact with tin- midbrain. In the primates, it lies in a subcallosal position, 

 resting upon tlu' roofplate in the ])ineal fossa of the midbrain. 



Such are the features on the external surface of the bram which consti- 

 tute an array of similarities sullicieiit to establish the ordinal solidaritx of the 

 ])nmates. Each of these distineti\-c features max not in all instances ha\e the 

 indul)itable quality of an idi'iitifying character. But the sum total of them 

 all serves as an entirely reliable guide for the ident ilication of any nu'inberot 

 the primate group. 



INDICAIIO.NS OF ENOLLTIONAI. UL\ ELOPMEN I IN THE PKI.MAIE BKAIN 



In connection with such distincti\'e features the question naturall}- 

 arises whether these charaett'rs are siibiect to such modifications as show 



