MOLLUSCA. To 
The heart is nearly in the middle of the back. Its auricle 
is on the right side, at the base of the branchiz; and the 
ventricle sends out at the opposite side three arteries. 
M. Cuvier has figured and described the P. Peronii with 
its anatomical details. Two species likewise appear to be 
known as natives of the British seas. 
Genus Burtia.—The head is destitute of tentacula and 
the body of the animal, protected by a convoluted shell, is 
oblong, becoming a little narrower in front. Below, the foot 
is broad, thin, and waved on the margin, expanded on each 
side behind, and capable of being turned upwards. At the 
posterior part of the foot, but separated from it by a groove, 
there is a broad, membranaceous appendage, a part of which 
is folded upwards, and a part spread over bodies, like the 
foot. It assists in closing the mouth of the shell, and in its 
position and use is analogous to the operculum, in the fol- 
lowing order. Above the foot, in front, also, but separated 
from it by a groove, there is a fat, fleshy expansion, which 
Cuvier terms the tentacular disc, considering it as formed 
by the union of the inferior and superior tentacula. In the 
centre of the disc, in the Bulla hydatis, (Lin. Trans., vol. 
ix. tab. 6, f. 4), Montagu observed two eyes. Between this 
portion of the back and the posterior extremity, is the dorsal 
plate or shell, forming the genus Bulla of conchologists. In 
some species, this shell is covered by the integuments, while 
in others it is exposed. But in all, the part containing it is 
partially concealed by the animal, by means of the reflected 
margins of the foot, and its appendage. Along the right 
side of the body there is a groove, formed by the foot and 
its appendage, on one side, and the dorsal plate and tenta- 
cular disc on the other. The branchize are situate in a 
cavity under the shell or dorsal plate, and resemble those of 
