166 



ted, as Hill has inaccurately stated. As a result of this peculiar 

 modification a deep olfactory cup is formed containing, apparently, 

 the ganglion of Jacobson's organ; the ganglionic structures placed 

 upon the actual surface of the lobe being related to the olfactory 

 nerves proper. In the natural condition of parts the cup is not very 

 evident because it is almost completely filled with the mass of enter- 

 ing nerves and blood vessels and is bridged over by a meningeal fold 

 (Fig. 5 men.). 



Professor C. J. Herrick describes ^) under the name of "mesal 

 olfactory fossa" a deep depression upon the mesial aspect of the 

 olfactory tuber of certain Ophidia (Tropidonotus, Coluber and 

 Eu taenia) and says that the structure so described is enormously 

 bigger in Serpents than in the Alligator and Opossum. He says that 

 "the localised development of pero on the ventral aspect is associated 

 with the isolated course of the highly developed tract of Jacobson's 

 organ". Professor Rabl - Kückhard describes a similar fossa in 

 Python molurus^). 



The olfactory cup in the Monotreme, although on the dorsal and 

 lateral aspects of the bulb, is apparently comparable to the "mesal 

 olfactory fossa" of the Reptile, since it appears to be definitely asso- 

 ciated with the development of Jacobson's organ. The cup in Platypus 

 is much deeper and more marked than in the Black Snake, where 

 Herrick describes it as enormous. This fact is interesting when one 

 remembers that the organ of Jacobson reaches its highest develop- 

 ment in the Monotreme. In spite of the high development of Jacob- 

 son's organ in Marsupials, there is no appreciable cupping in many. 

 In Perameles a faint mesial depression is to be distinguished, 

 which apparently corresponds to Herrick's "mesal fossa" in D i d e 1 - 

 phys. In Echidna, although I have not examined fresh material, 

 it is evident that the olfactory bulb difi'ers markedly from that of the 

 other Prototherian, although it has a highly developed organ of Jacob- 

 son. From these considerations, it would seem that the morphology 

 of the bulb of Platypus is largely determined by phylogenetic factors. 



Symington ^) has applied Turner's name "microsmatic" *) to 

 Platypus, and on the strength of this statement Wiedersheim ^) 

 applies the term to both Monotremes, including Echidna, whose 



1) Journal of Comparative Neurology, Vol 3, p. 78. 



2) Zeitschr. f. wiss. ZooL, Bd. 58, Htt. 4. 



3) loc. cit. 



4) Journal of Anatomy and Physiology, V. 25. 



5) loc. cit. 



