THE BRAIN IN THE EDENTATA. 317 



campal arc which we find in Marsupials and Monotremes *. la Orycternpus this circum- 

 callosal hippocampal vestige is a very ])rorainent rounded strand whicli we can readily 

 follow with the naked eye around the spleuiura, along tlie whole length of the corpus 

 callosum, and around its anterior extremity. But beyond this we cannot thus follow it. 

 We shall see that in most of the other Edentates the mode of termination of this 

 vestige of the anterior part of the hippocampal arc is more clearly exhibited than it is in 

 Orycteropiis. 



Turning to the consideration of the brain of Tamandua (fig. 10), we find a typical 

 hippocampal formation, three concentric bands of which — fimbria, inverted hippocamj)us, 

 and fascia dentata — appear upon the surface. These three bands end simply below, i. e. 

 without that peculiar modification which in Onjcteropus was described under the name 

 hippocampal tubercle. At its upper extremity we find the same series of circumsplenial 

 modifications of the hippocampal formation as we have already described above. But 

 the infrasj^lenial bending of the fascia dentata is much further removed from the 

 psa/terium ventrale than is the case in Orycteropus : in other words, the corpus callosum 

 has grown relatively further backward, carrying the infrasplenial hipjiocamjial flexure 

 {flexura hipp)ocampi) with it, and has extended the pi^^^ltermm dorsale to permit this 

 separation from the psaUerimn ventrale. 



A large supracallosal vestige of the hippocampus may be recognized upon the dorsal 

 aspect of the corpus callosum in Tamandua, but it is relatively smaller than the corre- 

 sponding vestige in Orycteropus. The deep cleft which separates the hippocampal 

 vestige from the overhanging pallium corresponds to the callosal fissure of human 

 anatomy. It is not, as is often erroneously stated, the continuation of the hippocampal 

 fissure, because the latter always ceases in the Edentates, as in most Eutheria, imme- 

 diately below the splenium of the corpus callosum when the hippocampus unrolls. The 

 hippocampo-pallial limiting furrow, Avhich emerges from the upper extremity of the 

 hippocampal fissure, surrounds the splenium, and joins the fissura cnllosalls, is often 

 mistaken for the hippocampal fissure itself, and hence the belief in the continuity of 

 hippocampal and callosal fissures. Unlike the condition of affairs in Orycteropus, the 

 Jissura callosalis in Tamandua becomes continuous anteriorly with a well-defined fissure 

 which arches downward and forward to disappear in the deep cleft which separates the 

 upper surface of the olfactory peduncle from the apex of the hemisphere. This is the 

 ventral boundary of the pallium, and hence may be called fissura limitans p)allii. It 

 serves to indicate to the naked eye the upper limit of the hippocampal vestige which we 

 can discover along this line by histological examination. 



In Myrmecophaga wo find a close agreement with the condition just described in 

 Tamandua. There is also a large and clearly-defined hippocampal vestige, which Forbes 

 has failed to indicate in his figure of tlie mesial surface. However, he speaks of the 

 fascia dentata " being continued, as described by Professor Turner in Dasypus, as a thin 

 layer of longitudinally-disposed fibres over the corpus callosum to near its genu" f. 



* Cf. this vol. p. 50 ; also ' Journal of Anatomy and Physiology,' vol. xssii. 

 t W. A. Forbes, op. cit., Proc. Zool. Soc. London, 18S-', fig. 4, and p. L'iJ4. 

 SECOND SERIES. — ZOOLOGY, VOL. VII. 4,^ 



