Houser, The Neurones of a Selachian. 145 
The forebrain of Mustelus appears superficially to be some- 
what divergent from the direct line of the series, owing to the 
absence of conspicuous external evidence of its bilaterality. 
The two striata are fused together into a solid basal mass; there 
is merely a broad and shallow furrow on the ventral side to 
mark their plane of contact. The pallium, while a thickened 
nervous plate as in far higher types of brains, is perfectly con- 
tinuous between the right and left sides, and it is also confluent 
in the median plane with the striata below. These peculiarly 
compacted characteristics have been the source of no little con- 
troversy. STUDNICKA has attempted to show as one of the 
dicta of a series of papers ('94a, ’94b, 95, 98) that the sela- 
chian forebrain stands entirely apart from that of all other ver- 
tebrates. His conclusions were drawn from studies on Petro- 
myzon, and they appear to rest upon a misconception as to the 
extent of the tela choroidea superior. The fallacies of his 
views have already been pointed out by BurcKHARDT (’94C¢), 
and by Rasi-RUcKHARD (’94). 
The lateral ventricles and the olfactory lobes of Mustelus 
really anticipate the two-lobed condition of higher forebrains 
in their essential characters. The lateral ventricles (Fig. 1) 
are derived from the outer angles of the third ventricle, and 
they pass the lamina terminalis some distance apart from each 
other. Their courses lie nearly parallel throughout. Each 
gives off two diverticula, the olfactory ventricle and a dorsal 
branch, respectively. The dorsal diverticulum, (Fig. 31, f. v.), 
is a short and narrow vertical cavity for the pallial eminence ; 
see Subsection 4. This structure, while peculiar to the brains 
of certain selachians, is nevertheless of some comparative value, 
as will be evident further on. 
The olfactory lobes are massive prolongations of the lat- 
eral angles of the forebrain. The primitive character of an 
olfactory lobe is exhibited by the sharply defined bulbus and 
tractus, and by the presence of the olfactory ventricle (Fig. 31, 
ol. v.). This is a slender cavity derived from the outer side of 
the lateral ventricle at the level of the pallial diverticulum. . 
The recessus neuroporicus of BuRCKHARDT (94a) is of 
