138 PHYSIOLOGICAL SERIES. 



inarkably long the distance between the olfactory bulbs 

 and the chiasina being very considerable, and the hinder 

 part- bring prolonged backwards over the entire dorsal 

 -urface of the optic lobes. As these parts are also very 

 deep dorso- vent rally the optic lobes are more depressed 

 than usual, and lie entirely upon the ventral surface. 



0. C. 1321 1. 



D. 188. The brain of a Bower-bird (Ptylonorhynchm violaceus). 

 The hemispheres are well developed, and completely cover 

 the dorsal surface of the optic lobes. They are relatively 

 broader and rounder than in the preceding specimen. No 

 furrows are visible upon their surface. The cerebellum is 

 comparatively small. 0. C. 1321 ia. 



MAMMALIA. 



ORDER MONOTREMATA. 



Family OHNITHORHYNCHIDA:. 



D. 189. The brain of a Duck-billed Platypus (Ornithorhynckut 

 anatinus); also a right cerebral hemisphere, dissected to 

 show the fascia dentata in the mesial wall. 



This brain, in common with that of the Spiny Anteater, 

 exhibits in the relative proportions of its various constituent 

 parts and in their degree of histological differentiation un- 

 mistakeable evidence of its conformity to the mammalian 

 type. But, on the other hand, the structural plan of several 

 important regions of the brain (notably of the cerebral 

 commissures and the neighbouring parts) differs in a very 

 pronounced manner from that which obtains elsewhere 

 among mammals (excluding the Marsnpialfl in regard to 

 certain of these features). It is significant that the general 

 arrangement of the " commis-tiral region/' which i- >o 

 peculiar in the Monotremes, essentially agrees with the 

 structural plan which is common to most non-mammalian 

 vertebrates. 



Many of then- peculiarities can be satisfactorily studied 

 only by histological examination, and lienee do not come 



