320 



DR. G ELLIOT SMITH ON 



TiK. 2-3. 



Three schemes to represent the relation of the hippocampal formation to the commissures in the 

 Mynnecoj^Jtcyidce (A), the Bradijpodidw (B), and the Basi/podida' (C). 



Reference Ictte 



e.c- 



.</•- 



spl- 



ps.v.- 



2^s.d.- 



a.c- 



l.t.- 



It'. 



p.- 



-Corpus eallosum. 



- Genu corporis callosi. 



-Splenimn corpioris calloni. 



-Psaltcrium venirale. 



-Fscdterium dorsale. 



-Commissura anterior. 



-Lamina terminalis. 



-Lamina terminalis (the intercommissural 



copidii). 

 -Area jiro'commissuralis. 

 -Area pracoinmissuralis (pars dorsalis =septum 



lucidum). 



f.d. — Fascia dentata. 



f.d'. — T\ie fascia dentata at ih.c Jle.vxu'a hippocampi. 

 Ji. — Fimbria. 



i.h. — The inverted hippocampus. 

 h.f.- — Fissura hippocampi, 

 n.h. — Uippocauipms nailiis^the exposed surface of 



the hippocampus proper. 

 v.h. — The vestiges of the hippocampus (supracallosal 



P'lrt). 

 v.h'. — Tlie vestiges of the hipjwcanipus (precallosal 

 part). 



In the Bradyiiodidce we have a peculiarly simjile arrangement of hij)poeampal 

 formation, which is all the more remarkable because it forms such a marked contrast to 

 the typically Eutherian type we have just found in the Ant-eaters. The small corpus 

 eallosum has produced a minimum disturbance in the circurasplenial parts of the 

 hippocampal arc. Tliere is practically uo flexura hippocampi, no appearance of the 

 naked hippocampus. On the other hand, the fascia dentata extends directly toward the 

 splenium and gradually tapers aud fades away without bending. The other changes i]i 

 this scheme are self-explanatory. 



We will now turn to the consideration of the hippocampal formation in the Armadillos, 

 the type of which is rejiresented in the third scheme (C). 



In all the Dasypodidce examined we find the tyjjical arrangement of the hij^pocampal 

 formation such as is common in the Eutheria. The A'estigial hippocampus {v.hS) is 

 relatively very large. 



In Xeniirus we have a beautifully clear demonstration of the arrangement of the 

 parts under consideration. The vestigial hippocampus is a plumj) arcuate cord, which 

 surrounds the diminutive corpus eallosum, and then bends downward and forward in 

 front of the commissure. 



It is equally prominent in Dasypiis sexcinctus aud Dasypus villosus (fig. 23, C) and in 



