FORM AND STRUCTURE OP SHELL. 365 



which binds together and moves the several segments in that 

 sub-kingdom. The material of which the shell is composed, 

 varies considerably in regard to the relative quantities of animal 

 and of calcareous matter which it includes. In the Cones, Cow- 

 ries, Olives, and others known as porcellanous shells, the quantity 

 of animal matter is so small, that it can with difficulty be 

 detected; but in others, as the Limpet, there is considerably 

 more ; and in some instances there is an almost entire absence 

 of calcareous matter, the shell having merely the consistence of 

 horn. It is remarkable that, notwithstanding so many of this 

 group are destitute of shell in their perfect condition, all the 

 species belonging to it possess the rudiments of a shell when 

 they come forth from the egg ; and this has nearly the same 

 form in every case, being usually a simple cone, with the point 

 slightly turned over. In most Gasteropods this shell is retained, 

 and is enlarged by successive additions ; but it soon falls off in 

 those cases, in which it is not to be permanent. The additions 

 which are made to the shell, for the purpose of deepening its 

 cavity and widening its mouth, sometimes appear to be confined 

 to the edge only ; whilst in other instances a new layer is thrown 

 out as a lining to the whole interior of the shell. In the former 

 case the line of junction between the old and the new parts is often 

 marked externally by a prominent rib ; but the internal surface 

 is beautifully smoothed off. 



908. The forms of the shell in this Class are subject to great 

 variations ; but those which appear most widely separated, may 

 be shown to be connected by intermediate links ; as well as to 

 have a common origin. The simplest of all shells in point of 

 form, is that of the common Limpet, which is merely a cone, 

 more or less expanded at the base ; and in which the successive 

 additions are all made in the same direction. In an allied genus, 

 the Pileopsis, we find the point or apex of the cone somewhat 

 prolonged and turned over, so as to resemble a " fool's cap" in 

 its form ; and the increase of this tendency produces a regular 

 spiral shell, such as that of the Planorlis, in which all the whorls 

 or turns are upon the same plane, as in a " Catherine's wheel." 

 But if the whorls do not continue in the same plane, but turn 



