GASTEEOPODA. 



205 



but more commonly the inner side of the spiral tube is foi-med 

 by the pre-existing whorls (Fig. 69). 



The axis of the shell, around which the whorls are coiled, is 

 sometimes open or hollow ; in which case the shell is said to bo 

 perforated, or umhilicated (e.g. solarium). The perforation may 

 be a mere chink, or fissure {rima), as in lacuna; or it may bo 

 filled up by a shelly deposit, as in many naticas. In other shells, 

 like the triton, the whorls are closely coiled, leaving only a 

 pillar of shell, or columella, in the centre : such shells are said 

 to be imperforate. 



\ apex. 



► spire, 

 suture. 



I posterior canal. 



outer lip 



>■ of the 

 aperture. 



./ anterior canal. 



Fig. 69. Section of a spiral univalve.* 



The apex of the shell presents important characters, as it 

 was the nucleus or part formed in the egg ; it is sinistral in 

 the pyramidellidce, oblique and spiral in the nucleohranchs and 

 emarginidce, and mammillated in Turhinella pyrum and Fusua 

 anti(]uus. 



The apex is directed backwards in all except some of the 

 patellidce, in which it is turned forwards, over the animal's head, 



* Fig. 69. Longitudinal section of Triton corrugatus, Lam., from a specimen in tlie 

 cabinet of Mr. Gray. The upper part of the spire lias been partitioned off many limea 

 Biiccessively. 



