PARTS OF THE UNIVALVE SHELL 



263 



whorls. It may he deep, unci even channelled, or very shallow, 

 as in ¥ig. 150 B (p. 24 G). 



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

 hod//, '(nhorl. A tahorl is a single revolution of the spiral cone 

 round the axis. The spire may l:)e sithulate (as in Terebra, Fig. 

 150 C), turi'etted (Scalaria), depressed {Polyyyratia, Fig. 150 A), 



apox 



varix . r 

 umbilicus . v ^% 



dilumella 

 anteriiir canal 



posterior canal (if present) 

 outer lip 



mouth or aperture 



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

 imivalve shell. 



conical (Trochns'), f/lohose (Ampnllaria, 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 {a) entire, as in Helix, 

 Natica, Amiyullaria, when its peristome or margin ^ 



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

 prolonged at its anterior and sometimes also 

 at its posterior end into a cannL The anterior 

 caned serves as a protection to the siphon,^ the 

 posterior caned, is mainly anal in function, and 

 corresponds, in part, to the hole of Fissurella, 

 the slit in Pleurotoma and Emarginula, and the fig. 175.— Anal 

 row of holes in Haliotis. The mouth presents slit m Pleim- 



'■ _ toma. 



every \-ariety of shape, from the perfect circle 



in Cyclostomet and I'rocJiUS, to the narrow and prolonged slit in 



Conns and Olivet. 



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



^ Sfromhi.'i anil Pfcrocarcs (see Fig. 99, p. 200) excejitionally develop a siplional 

 notcli which is distinct from tho anterior canal. 



