470 



PROF. G. ELLIOT SMITH ON THE MORPHOLOGY OF THE 



undoubted identity of arrangement is to be attributed to a close genetic relationship, 

 or to an evolutionary convergence, does not come within the scope of this enquiry : it is 

 sufficient that the homology is so clear as to be indisputable. 



Of a large number of Sauropsidan brains which I have examined, I have chosen that 

 of a common Australian Hydrosauriis, because it lends itself so admirably to exact 

 comparison with the Monotreme brain. 



If a section be made through the brain of this Monitor in a plane corresponding to 

 that which was made through the Prototherian brains which have just been considered, 

 a state of affairs will be found which it is easy to interpret after comparison with the 

 foetal and adult Monotreme arrangement. 



In this section (fig. 18) two commissural bands are found dis2)osed according to a plan 



Coronal section of cerebral hemis])heres of Hiidrusiniriis. 



which is analogous to that of the Monotreme. The question whether these dorsal and 

 ventral commissures are not merely analogous but also homologous to the corresponding 

 commissures in the Monotreme will l)e discussed subsequently, but I may for the present 

 call attention to the similarity of their disposition. 



I may remark, m passing, that no writer who has treated of this question, even though 

 the conclusions at which he arrives are diametrically opposed to my own, has refused to 

 admit the fact of the correspondence in position of these commissui*al bands in the Reptile 

 and Monotreme. [As an example, I might refer to the writings of Osborn, ' Morpholo- 

 gisches Jahrbuch,' Bd. xii. p. 223, and numerous other instances might be quoted.] 



Upon the upper surface of the ventral commissure, in the section under consideration, 

 two bulky masses of grey substance are found joined across the middle line by bridges of 

 grey substance. These grey masses are the posterior extremities of the paraterminal 

 bodies, which are relatively much larger than the corresj)onding bodies in the Mono- 

 tremata. In the next figure they are represented upon a larger scale. Upon the upper 

 surface of the paraterminal body a recessus superior is found resembling in all its features 

 that of the adult OrnUhorlvjnclms. Thus it presents an epithelial roof, invaginated to 

 form a mesial choroid plexus — " dlaplexxis " of Wilder. Its lateral walls are thickened 



