MOLLUSC A. 175 



The heart is nearly in the middle of the back. Its auricle 

 is on the right side, at the base of the branchiae ; 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 BULLA. 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 branchiae are situate in a 

 cavity under the shell or dorsal plate, and resemble those of 



