UNIVALVE SHELLS. 



481 



curled ; others made like an hollow gutter or pipe : some 

 fashioned as it were a comb, others waving with plaits 

 one aboue another tile-wise, others framed in the manner 

 of a net or lattise : some are wrought crooked and byas, 

 others spred out directly in length. A man shall see of 

 them those that are made thick and mossie thrust together 

 and compact, others stretcht forth at large : ye shall 

 haue of them wrapt and lapt one within another : and to 

 conclude, yee shall find them run round into a short fast 

 knot, and all their sides vnited together in one : some 

 flat and plain, good to giue a clap, others turning inward 

 crooked like a cornet, made as it were to sound and wind 

 withall." * 



The apex is to be distinguished almost always by a diffe- 

 rence in its colour from the other whorls : it is often horn- 

 coloured, and always unicolorous. It becomes concealed 

 more or less completely in some convolute shells when these 

 have attained maturity. This is observable in some Volutes 

 and in the Cowries (Fig. 97). 



The imaginary internal axis around which the whorls re- 

 volve, is named the pillar or columella (Fig. 99). When 

 solid and close at the base, the shell 

 is imperforate ; when open, the base 

 is umbilicate. The umbilicus is well 

 exemplified in Trochus and Natica; 

 and still more remarkably in the genus 

 Solarium. It is found only in such 

 shells as have an entire aperture. 



At maturity, the umbilicus is occa- 

 sionally obliterated by the normal cha- 

 racter of growth of the shell, as in many 

 Helices; or by a deposition of shelly 

 matter. It is then said to be obtected, 

 or covered by a callus. This is very 

 general in Natica. 



The aperture occupies a more or less 

 considerable portion of the body-whorl, 

 and is limited to it. 



When the margin is entire or, as it 

 were, unbroken or chipped, the aper- 

 ture is holostomatous ; but if the margin 

 is interrupted by a sinus, or gutter, or 

 canal, it is siphonostomatous. If the 

 margin shows only a wide and shallow depression at its base 



Fig. 99. 



Hist, of the World, i. 253. 



1 I 



