100 



MANUAL OF THE MOLLUSCA. 



in contact, as scalaria, cijclostoma, and valvata ; bnt more commonly the 

 inner side of the spiral tube is formed by the pre-existing whirls (fig. 62). 



The axis of the shell, around which the whirls are coiled, is sometimes 

 open or hollow ; in which case the shell is said to be perforated, or umhili- 

 cated (e, g. solanuvi) , The perforation may be a mere chink, or fissnre {iiam), 

 as in lacuna ; or it may be tilled up by a shelly deposit, as in many naiicas. 

 In other shells, like the tritan, the whirls are closely coiled, lea^dng only a pillar 

 of shell, or columella, in the centre ; such shells are said to be imperforate. 



"" "> apex. 



t> spire, 

 suture. 



J posterior canal. 



outer lip 



of the 

 apertm-e. 



— -_; anterior canal. 



Fig. 62. Section of a S'piral univalve* 



Tlie apex of the shell presents important characters, as it was the nucleus 

 or part formed in the egg ; it is sinistral in the ipyramtdeUidce, oblique and 

 spiral in the nucleohranches and emargimdce, and mammillated in turhinella 

 pi/rum anifusus antiquus. 



The apex is directed backwards in all except some of the patellida, in which 

 it is tm'ned forwards, over the animal's head. In the adult condition of some 

 shells the apex is always truncated (or decollated), as in cylindrella and buli- 

 mus decollatus I in others it is only truncated when the animals have lived 



■^ Fig. 62. Longitudinal section of triton corrugatus, Lam., from a specimen in the 

 cabinet of Mr. Gray. The upper part of the spire has been partitioned off many times 

 successively. 



