280 SYDNEY E. JOHNSON 
In comparing the lateral sensory canals of Mustelus canis 
and Squalis sucklii there are a number of differences to be noted. 
Perhaps the most striking is the difference in calibre of the sensory 
tubes. The sensory tubes (or canals) of Squalus are much 
smaller than would be found in a Mustelus specimen of the same 
size. The column of sensory epithelium is_ proportionately 
narrower in Squalus. A slight but apparently constant differ- 
ence in the course of the lateral canals of the two species is seen 
in the slight elevation of the canal above the anal fin in Mustelus. 
There are other differences in the distribution of the canals, but 
they are less striking and have not been carefully examined. 
The lateral canals of both species lie chiefly in the dermis and 
their tubules pass directly ventrad for a short distance before 
making a sharp bend laterally to open on the surface of the integ- 
ument. The surface tubules correspond in number with the 
ramuli of the lateral nerve and there are approximately five 
tubules for every four segments of the vertebral column. 
The lateral nerve lies at a considerable depth from the sensory 
canal, especially in the anterior region, and its ramuli pass 
obliquely to the basilar membrane of the sensory column, where 
their fibers diverge caudad and cephalad to form a continuous 
longitudinal fiber zone just outside of the basilar membrane. 
This fiber zone differs from that described for Mustelus only in the 
fact that it contains a considerably smaller number of nerve 
fibers. 
The sensory epithelium of Squalus sucklic differs considerably 
from that of Mustelus canis. It is much less extensive and the 
sensory cells are aggregated in smaller groups. This can be 
seen readily in transverse and longitudinal sections. In the 
former one to three sensory cells can ordinarily be seen in the 
cell clusters (fig. 1), and in the latter, usually three to six 
(figs. 2 and 10). The groups of sensory cells are somewhat more 
widely separated from each other than they are in Mustelus, 
and the sensory column appears to show a stronger tendency 
towards segmentation. This apparent segmentation of the 
column of sensory epithelium, however, bears no relationship 
to the normal body segments for there are usually more than ten 
