NERVOUS SYSTEM. VERTEBRATA. 371 



mammals. [The specimens of Myrmecophaga are especially 

 instructive in regard to this point.] 



The form which the calcarine sulcus has assumed in this 

 brain may raise some doubt whether the anterior limb of 

 the calcarine sulcus should not be regarded as the strict 

 homologue of the parieto-occipital sulcus of the Anthro- 

 poidea. It must be admitted that the length of the upper 

 limb of the Y, its wide separation from the other post- 

 calcarine limb, and its relation (on the dorsal surface) to 

 the intraparietal sulcus (especially on the right hemi- 

 sphere), closely simulate the corresponding relations of 

 the parieto-occipital sulcus in the Apes. Its wide separa- 

 tion from the intercalary (calloso-marginal) sulcus serves 

 to accentuate the likeness. 



When the cerebral hemisphere begins to extend into a 

 caudal or occipital process, as it does in the Lemurs, and 

 to a much more pronounced degree in the Anthropoidea, 

 the calcarine sulcus assumes an increasingly oblique position 

 and blends with certain postcalcarine elements (which occur 

 in most mammalian Orders). These postcalcarine elements 

 are morphologically very unstable and inconstant in their 

 arrangement (compare the larger Carnivores and Ungulates, 

 and also the Apes), and tend to adapt themselves solely to 

 the mechanical conditions which prevail. Thus in the 

 Carnivora they are generally parallel to the calcarine, 

 because the tendency to caudal extension is restrained : in 

 the Primates this is not so, and the sulcus runs in the long 

 axis o the occipital diverticulum (compare Midas or 

 Hapale). But in many forms (compare Manis and Dauben- 

 tonia) a second element, simulating the parieto-occipital 

 of higher forms, develops to relieve the tension of the 

 growing cortex in that region, which is accommodated in 

 most mammals by the fusion of the calcarine and inter- 

 calary sulci. The condition found in Daubentonia makes it 

 abundantly clear that this is so ; and we can have no hesi- 

 tation in regarding the state of affairs in the Lemurs as 

 essentially identical with that of the Aye-aye, however 

 closely the former may simulate the condition sometimes 

 found in the human brain. But just as we know that the 



