DESCRIPTIVE REMARKS. XXX1X 
Fig. 3.—A UNIVALVE SHELL. 
Class GASTEROPODA, Buccinum undatum (recent). 
The Whelk. 
Original reduced to two-thirds of natural size. 
A.—Section of under side, a. p. Aperture. 
a. Apex. e. a. Canal. 
w. Whirls of spire. B.—Upper side-back. 
s. s. Sutures. ce. c. Coste. ; 
ce. c. Columella. t. s. Transverse strie. 
o. m, Outer, or right margin. C.— Operculum, lamellar. 
n. Nucleus (external). 
7. m. Inner, or left margin. 
The Gasteropoda are of land, fresh water, and marine habitats. 
They are usually called ‘ univalves,” such as land snails, whelks, 
limpets, &c. The shell is, however, sometimes composed of several 
pieces, and is then termed multivalve, as in chiton; in many cases 
there is noshell, or merely a rudimentary one, as in the slugs. 
