IN THE GENUS SQUALUS. 
145 
In the older fish, the intermediate space in the depres- 
sion between the cuticle and the cranium is chiefly filled 
with a transparent gelatinous substance, which can easily 
be seen and felt in the recent subject, by passing the finger 
backwards over the depression, when the posterior part will 
become tumid like a bladder. In the posterior and rather 
inferior and external parts of the depression are four fora- 
mina, two of which are situate on each side of the posterior 
process. The foramen next to the process is large, and of 
a circular figure, and, in the recent subject, covered with a 
membrane, the plane of which forms an angle of nearly 45°, 
with a line through the centre of the cranium, and about 
the same angle from the perpendicular. This membrane 
may with propriety be termed the Membrana labyrinthi. 
The spongy, elastic, membranous substance is attached to 
the external side of the membrane ; but, in the Skate, there 
is a considerable space between it and the substance. This 
large foramen leads into the vestibule and posterior semi- 
circular canal, and may be termed the Foramen rotundum, 
from its circular figure. Close to, and outside of this fora- 
men, is a very oblong aperture, through which the tube of 
the ear, in the young subject, enters the vestibule, and 
which, from its extreme oblong figure, may be termed the 
Foramen oblongatum. 
In the young subject of the species S quoins canus, the 
meatus auditorius externus is situate on the superior and 
posterior or coronal surface of the head, is small, and much 
contracted, and seldom admits of any substance larger than 
a strong bristle. The tube is formed of tough, elastic, mem- 
branous substance, and runs outwards or laterad, forwards 
or antinead, and downwards or basilad, a few lines, more 
or less, according to the size of the animal, until it reaches 
within a short space of a membrane stretched across the 
tube, where it enlarges to nearly twice the size of the other 
VOL. VI. K 
