1 86 MOLLTJSCA. 



perforation in the apex of the Shell, and likewise the water 

 which enters the branchial cavity in front. The head is fur- 

 nished with two tentaculae, bearing the eyes at the external 

 base. The Patella gr&ca 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 en- 

 trance to the branchiae and anus. The foot is surrounded 

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

 footstalks, characters in which it approaches the genus Hal- 

 yotis. The Patella Jissura 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 group 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 

 unnecessary. They are either oviparous, or ovoviviparous. 

 The presence or absence of a shelly covering, furnish cha- 

 racters for a twofold distribution of the groups. 



SECT. 1. Acephala Conchifera. 



The shell in all cases is external and bivalve ; and ex- 

 hibits very remarkable differences in the form, relative size, 



