THE BRAIN IN THE EDENTATA. 321 



Tolypeutes ; and in the small brain of ChlamydopJwrits (fig. 16) we again find an 

 admirable demonstration of the arrangement of this archaic hippocampal arc. In the 

 small brain of Chlaniudophorus I have traced in a complete series of sections the series of 

 changes in the hippocampus as it surrounds the spleniiim of the corpus callosum and 

 joins the su2:)racallosal vestige, and found that they agree in all points with the transition 

 region in such simple Eutherian brains as those of Erinaceus. These changes need no 

 description if we compare the schemes C and A in fig. 23. 



hippocamp. vestigia 



fimbria 

 ^'\faspia dentata 

 bulb, olfact.i- V-y yr^%^'tip5fean.pi 



hippocamp. inverBua 



area praecommisa. : 



taberoal. olfact. 



Mesial surface of right cerebral hemisphere of Xeniu-us uniciiictus. Nat. size. 



In Man is we find an exactly analogous arrangement. The large vestigial hijjpocampus 

 Avhich I found in my specimen is not shown in Mas Weber's figure {loo. cit.), but 

 probably goes to swell the thickness of the corpus callosum. 



In the preceding pages I have entered into considerable detail concerning the exact 

 disposition of the hippocampal formation in Orijcteropus and the Ant-eaters for several 

 reasons. 



In the first place, if the Ant-eaters occupy the lowly position near the bottom of the 

 mammalian phylum which is usually assigned tliem, we might expect some indication 

 of this fact in that region of the brain which has undergone such a striking modification 

 Avithin the Eutheria, namely in the hippocampal formation. Instead of this, Ave find the 

 typical Eutherian plan in as fully developed form as that of the Ungulata and 

 Carnivora. 



In the Sloths, it is true, we find an arrangement of hippocampus which is as simple as 

 it is pos>il)le to be in the presence of a corpus callosum. But the resemblance of the 

 condition in the Sloths to that which we have found in certain small bats *, and which 

 is obviously a reversion-type, suggests that the simplicity of the Sloth brain may possiljly 

 not l)e primitive. At the same time Ave must recognize the possibility of the archaic 

 simplicity of type being retained, just as it is, for instance, in the case of the auditory 

 ossicles according to Kitchen Parker. In the Armadillos, again, we have the typical 

 Eutherian plan exemplified. 



The relatively large vestigial hippocampus in these Edentata invited a minute study 

 in the hope of finding some definite indications of its original structure; but here again 



* Cf. this vol. p. 58. 



