PARTS OF THE UNIVALVE SHELL 



263 



whorls. It may be deep, and even channelled, or very shallow, 

 as in Fig. 150 B (p. 246). 



The spire is the whole series of whorls except the last or 

 body ivhorl. A luhorl is a single revolution of the spiral cone 

 round the axis. The spire may be subulate (as in Terebra, Fig. 

 150 C), turret ed (^Scalaria)^ depressed {Folygyratia^ Fig. 150 A), 



apex 



' spire 



— ' posterior canal (if present) 

 .. outer lip 



'*• mouth or apertiire 



Fig. 174. — Illustrating the technical terms applied to the various parts of a 

 univalve shell. 



conical (^Trochus')^ globose (^Ampullaria, Natica^ Fig. 150 B), 

 with almost all conceivable gradations between these types. 

 The number of whorls is best counted by placing the shell 

 mouth downwards, and reckoning one for every suture that 

 occurs between the extreme anterior point of the shell and 

 the apex. 



The mouth or aperture may be (<x) entire, as in Helix^ 

 Natica^ Ampullaria^ when its peristome or margin 

 is not interrupted by any notch or canal, or (5) 

 prolonged at its anterior and sometimes also 

 at its posterior end into a canal. The anterior 

 canal serves as a protection to the siphon,^ the 

 posterior canal is mainly anal in function, and 

 corresponds, in part, to the hole of Fissurella, 

 the slit in Pleurotoma and Emarginula^ and the 

 row of holes in Haliotis. The mouth presents 

 every variety of shape, from the perfect circle 

 in Cyclostoma and Trochus^ to the narrow and prolonged slit in 

 Conus and Oliva. 



The right margin of the mouth (the left, in sinistral shells) 



1 Strombus and Pteroceras (see Fig. 99, p. 200) exceptionally develop a 

 siphonal notch which is distinct from the anterior canal. 



Fig. 175. — Anal 

 slit in Pleuro- 

 toma. 



