826 DE. G. ELLIOT SMITH ON 



given off, \^ liicli are probably the expression of a puckering of the cortex at the seat of 

 flexure. 



The posterior ascending part of the sulcus is A^ery deep, and indents the whole 

 thickness of the cortex, i. e. is a " complete fissure.'" The possible significance of this 

 observation will be discussed later. 



The arrangement of sulci upon the mesial surface of the hemisphere in 3fi/rmecophaga 

 has been well represented by Forbes*. The extensive sulcus a of Orijcteropus is 

 represented by an analogous arrangement of three independent sulci: — a posterior 

 vertical {cm" of Forbes), Avhich we may distinguish as a' ; a longitudinal {cm of Forbes), 

 which we may call a" ; and an oblique anterior {cm of Forbes), which we may call a'". 

 The longitudinal sulcus (a") extends beyond the corpus callosum at each end, and sejoarates 

 the supracallosal pallium into t^^o longitudinal strips. 



The po.sterior vertical sulcus («') appears to l)e much more extensive than the cor- 

 responding 2)art of the sulcus a in OriicteropHS. This apparent ditference serves to 

 accentuate the constant relationship which the sulcus a maintains to the fissiira rliinalis. 



In Ilp'mecophaga the rhinal fissure terminates at the ventral border instead of high 

 up on the posterior border of the hemisphere as it does in Orycteropus, and the sulcus a' 

 descends to the ventral limit of the pallium, i. e. in close proximity to the fissura 

 rhinalis. 



In Tamandiia there is no definite sulcus representing the a scries of ITi/rmecopha(ja,hxiii 

 merely three deep pit-like depressions, which are probably the rudiments of the cor- 

 responding three sulci in the Great Ant-eater. The sulcus a! is represented by a short 

 deep sulcus, merely 3 mm. long, which rises as high as the level of the splenium of the 

 corpus callosum. The sulcus a"' is represented by a deeply-incised triangular depression 

 a short distance in front of the genu of the corpus callosum. Above the anterior 

 extremity of the corpus callosum there is a short shallow furrow which is partially 

 analogous to the sulcus a" in the Great Ant-eater. 



In the figure of the mesial surface of the brain of C//cloturus which Pouchet gives t 

 there is no trace of any pallial sulcus. 



In the Bradi/podidce the mesial surface of the pallium presents an arrangement of 

 sulci which is analogous to the a-system in the Ant-eaters. In addition the mesial 

 extremity of a sulcus, which belongs more especially to the cranial aspect of the pallium, 

 extends on to this surface of the hemisphere in both Cliohtpns and Bradi/pus. This 

 sulcus, which we shall describe later under the designation {6, deeply notches the anterior 

 border of the hemisphere. 



A sulcus, which we may distinguish as a, begins upon the mesial surface of the 

 hemisphere in Cliolcepus didaciylus, just in front of the termination of the posterior rhinal 

 fissure, and opposite the midpoint of the hippocampal fissiire. Its relation to the rhinal 

 fissure is exactly similar to the sulcus we have similarly designated in Myrmecophaga. 

 As it ascends it j)ursues a course which is approximately j)arallel to the upper half of 



* Forbes, op. cH., Proc. Zool. Soc. Londou, 1SS2, p. 293, tig. 4. 

 t Pouchet, ojj. cif., torn. vi. pi. iv. fig. 4. 



