752 Mr Buchanan on the Comparative Anatomy of the 
sentially necessary to the perfect ear, the semicircular canals are 
of almost incredible extent. 
The cranium being composed of semi-transparent cartilagi- 
nous substance, of a beautiful cerulean hue, the whole of the or- 
gan can be distinctly seen, by merely removing the cuticle, and 
some of the surrounding parts. 
When the cranium is divested of the common integuments, a 
considerable depression is seen in the coronal and posterior por- 
tion, which, in the living subject, is occupied by a kind of spongy 
elastic cellular membrane. 
This depression is of a heart-like figure, the apex pointing 
forwards, with a roundish process in the posterior part, which 
causes a partial division of this portion of the cavity. 
In the older fish, the intermediate space in the depression be- 
tween the cuticle and the cranium, is chiefly filled with a trans- 
parent 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 foramina, two of which are situated 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 center of the cranium, and about the same angle from the 
perpendicular. This membrane may, with propriety, be termed 
the Memhrana Labyrinthi . 
The spongy elastic membranous substance is attached to the 
external side of the membrane ; but in the skate there is a consi- 
derable 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 foramen, is a very oblong aper- 
ture, 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 . canus , the meatus au- 
