186 MOLLUSCA. 
perforation in the apex of the shell, and likewise the water 
which enters the branchial cavityin front. The head is fur- 
nished with two tentacule, bearing the eyes at the external 
base. The Patella greca 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 branchiz 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 fissura of conchologists is considered 
as the type of the genus. 
Diviston I]l.—Monutusca 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 branchiz. 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, 
4 
pe aie 
