MOLLUSCA. 495 



It is probable that the species here alluded to is the same 

 with the Pterotrachea coronata of FORASK^L. 



Family 2. Fissurellada. Shell with a slit, or perforation. 



In the Fissurella, the apex of the shell is perforated, 

 and united to the cloak by a circular muscle open in front. 

 The cloak forms a duplicature in front for the branchial 

 cavity, which extends to the perforated apex of the shell. 

 The gills consist of two ridges ; at the dorsal extremity of 

 which is the anus. It is probable that the excrements are 

 ejected at the perforation in the apex of the shell, and like- 

 wise the water which enters the branchial cavity in front. 

 The head is furnished with two tentacula, bearing the eyes 

 at the external base. The Patella grceca and apertura 

 may be quoted as British examples of the genus. 



The genus Emarginula differs from the former in the 

 apex of the shell not being perforated. Its place, however, 

 is supplied by a slit on the anterior margin, which is the 

 entrance to the branchiae and anus. The foot is surround- 

 ed with a row of filaments, and the eyes are supported on 

 short foot-stalks, characters in which it approaches the ge- 

 nus Halyotis. The Patella Jtssura of conchologists is 

 considered as the type of the genus. 



DIVISION II. 

 MOLLUSCA ACEPHALA. 



Destitute of a distinct head, or neck. 



The animals of this division are much more simple in 

 their organization than those of the preceding division. In 

 none of the species are there any rudiments of organs of 

 hearing or of sight. They are destitute of jaws or other 

 hard parts about the mouth. They all inhabit the water, 

 and possess branchiae. The organs of the two sexes are in- 

 corporated in the same individual, and reciprocal union is 



