TOO 



MA^'UAL OF THE MOLLUSCA. 





in contact, as scalaria^ cyclostoma, and valvata ; but more commonly the j 

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

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

 opeu or hollow ; in which case the shell is said to be perforated, or umbili- j 

 rated (e, g. solarium). The perforation may be a mere chink, or fissm-e {nam)^ \ 

 as in lacuna ; or it may be filled up by. a shelly deposit, as in many naticas. i 

 In other shells, like the triton, the whiiis are closely coiled, lea%dng only a pillar \ 

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



■-■^ apex. 



suture. 



canal. 



J 



J anterior canal. 



Fig. 62. Section of a spiral univalve.* 



The apex of the shell presents important characters, as it was the nv.cleus 

 or part formed in the egg ; it is sinistral in the pi/ramidellidcB, oblique and 

 spiral in the nucleobranches and emarginulce, and mammiUated in turbinella 

 pyrum andfusus antiqmcs. 



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

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

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

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



^ Fig. 62. Longitudinal section of triton cnrrugatus, Lam., from a specimen in the I 

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

 successively. 



