NERVOUS SYSTEM. VERTEBRATA. 219 



makes its appearance ; whereas in Cholcepus, in which the 

 neopallium has not extended downwards in its caudal region 

 (witness the horizontal posterior rhinal fissure), there is no 

 " Sylvian fissure " even though the cerebral hemisphere is 

 larger and therefore the greater a priori reason for a 

 Sylvian fissure. 



The extensive and deep suprasylvian sulcus (fig. 92, 8) is 

 also much less acutely flexed than is the corresponding 

 feature in Bradypus. On its ventral side a short horizontal 

 sulcus makes its appearance to compensate for the absence 

 of the Sylvian element, thus affording further evidence of 

 the mechanical mode of formation of the so-called " Sylvian 

 fissure " of Bradypus. 



The presylvian or orbital sulcus (/3) joins the rhinal fissure 

 in both hemispheres; otherwise it resembles that of Brady- 

 pus. On the left side it joins or becomes confluent with the 

 rostral sulcus, but on the right side it terminates near the 

 mesial edge just behind the upper extremity of the rostral, 

 which is a small independent sulcus. 



There is an extensive, sagittal, lateral sulcus, which bifur- 

 cates posteriorly. In this specimen (fig. 91) the lateral 

 sulcus is fused with the coronal, but in that from which 

 figure 92 was drawn these two elements were independent 

 (7 and y) . There is also a well-defined entolateral sulcus 

 i. e. a compensatory sulcus developed on the mesial side 

 of the lateral. 



The calcarine sulcus is peculiar in the Sloths in that it is 

 independent of the intercalary sulcus (with which it is fused 

 in most mammals to form the so-called " splenial " sulcus 

 of Krueg). In this respect the Sloths agree with the true 

 Anteaters, the Pangolins, and the Primates. This form of 

 the calcarine sulcus is undoubtedly the primitive, ancestral 

 type for all mammals. 



The calcarine sulcus is vertical and terminates near the 

 upper margin of the hemisphere (fig. 93, a). 



The intercalary sulcus begins very far back between the 

 splenium of the corpus callosum and the upper part of the 

 calcarine sulcus. On the left side it extends horizontally 

 forward almost as far as the rostral sulcus. On the right 

 side its anterior extremity bends upward and notches the 



