168 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
elongate and the lobe is not differentiated into bulb and tract. The 
nerve in all forms consists solely of special sensory fibres, and its ap- 
parent origin from either the tip or the ventral surface of the cerebrum 
ED 
Fe SE 
B 
Fic. 168.—Diagrams of the different kinds of olfactory bulb, tract, and nerve. 4a, ol- 
factory bulb; g, glomeruli; ol, olfactory lobe; on, olfactory nerve; to, olfactory tract. 
Fic..169.—Brain and olfactory and (nf) terminalis nerves of Raia, after Locy. 
is to be explained by the varying development of the hemispheres as 
given above (p. 159). 
In some fishes (several elasmobranchs, dipnoi, Amia) a small 
nerve arises dorsally (some elasmobranchs) or ventrally from the cere- 
brum, has a distinct ganglion and, following somewhat closely the 
