GASTEROPODA. 205 
but more commonly the inner side of the spiral tube is formed 
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 be 
perforated, or wmbilicated (e.g. solarium). The perforation may 
be a mere chink, or fissure (rima), as in lacuna; or it may be— 
filled up by ashelly deposit, as in many naticas. In other shells, 
like the triton, the whorls are closely coiled, leaving only a 
pillar of shell, or colwmella, in the centre: such shells are said 
to Le imperforate. 
\ spire. 
suture. 
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 pyramidellide, oblique and spiral in the nucleobranchs and 
emarginule, and mammillated in Turbinella pyrum and Fusus 
antiquus. 
The apex is directed backwards in all except some of the 
patellide, in which it is turned forwards, over the animal’s head. 
* Fig. 69. Longitudinal section of Zriton corrugatus, Lam., from a specimen in the 
cabinet of Mr. Gray. The upper part of the spire has been partitioned off many times 
successively, 
