14 PROCEEDINGS OE THE SCIENTIFIC ASSOCIATION* 
C. Lamellibranchiata. 
Unionidse [AnodonJ 1 species. 
I take the present opportunity of acknowledging my 
obligations to Mr. Robert Swift, of St. Thomas, a gentle- 
man whoso great knowledge of the mollusca of the West 
Indies is well-known, and whose assistance has been on 
sevoral occasions of the greatest service to me. 
StJBKINGDOM MOLLUSCA. 
Class Gasteropoda. 
This class includes all the Mollusca in which the under 
Burface of the body is formed into a single muscular foot or 
creeping disk. The shell is usually a single spiral tube, as 
in the land-shells. In the slugs the shell is rudimentary 
or quite wanting, and one large subclass (Nudibranchiata) 
of sea-slugs is not furnished with a shell except in the lar- 
val condition. The limpets have a shield-shaped shell, 
and Chiton is provided with one composed of eight imbri- 
cating plates, while the tooth-shells ( Dentalium ) are tubu- 
lar and straight or slightly curved. 
Most of the shell-bearing Gasteropoda are furnished with 
an operculum or lid which closes the aperture of the shell 
when the animal is retracted. In some genera and one 
whole subclass this appendage is wholly wanting. 
Subclass Inoperculata, 
Shell generally completely covering the animal, or some- 
times rudimentary or wanting. Oporculum wanting. 
Animal broatliing air ; lingual teeth very numorous, si- 
milar, on a broad dental ribbon.* 
* Cylindrella (trinitaria) has a long and narrow dental membrane 
with tew teeth iu each row* 
