476 TERMINOLOGY OF 



columella, is, therefore, interposed, which supports the vis- 

 cera, and separates them from the locomotive organ." * 



III. SPIRAL OR TURBINATE UNIVALVES COCHLEAE, LIN. 



The shells of the Snail and Whelk are examples of spiral 

 univalves. When they cover the body no adjective term is 

 needed to express their position, but when they are imbedded 

 in the mantle, the shell is said to be internal, and the Mol- 

 lusk is said to be naked. All internal shells are white or 

 horny, and they are only obsoletely spiral. 



The annexed figure (Fig. 96) is that of a 

 spiral univalve, in which a is the apex or 

 tummit, s the spire, o the aperture, and b 

 the base. 



The spire consists of one or more whorls, 

 a whorl being a complete revolution of the 

 shell round its axis or columella. The shell 

 figured has seven whorls. 



From modifications produced by the plane 

 on which the whorls revolve, the following 

 figures are derived, 



Discoid. When the whorls revolve on a 

 horizontal plane and are applied close to 

 each other, a flat or disc-like shell is the 

 necessary result. In its volutions the shell 

 enlarges insensibly from the centre or point 

 of departure, and hence it follows that every whorl is larger 

 in all its dimensions than the preceding one, and the centre 

 itself is sunken on one or both sides. The sunken or 

 depressed side is said to be umbilicate, when the depression 

 is considerable. Ex. Planorbis. 



Cylindric. When the whorls are nearly equal in diameter 

 and rise on each other without any marked tapering. Ex. 

 Pupa. 



Conical or pyramidal. When the base is broad and flat, 

 while the whorls form a spire graduated to a point. Ex. 

 Trochus. ..: 



Turbinate. When the whorls rapidly decrease in size and 

 diameter, and form a conical oblique spire longer than the 

 diameter of the body whorl. Ex. Littorina. (Also Fig. 96.) 

 Globose. When the whorls are few and scarcely raised 

 above the body, so that all the diameters of the shell are 

 nearly equal. Ex. Dolium, Helix. 



* Trans. Zool. Soc. Lond. i. 210. 



