512 



MOLLUSCOUS ANIMALS GENERALLY. 



Fig. 257. 



of most bivalve shells, is occasionally traversed by tubes, which 

 seem to commence from the inner surface of the shell, and to 

 pass towards the exterior. These tubes vary in size from about 

 the l-20,000th of an inch, or even less, to about_ the l-2000th ; 

 but their general diameter, in the shells in which they most 



abound, is about l-4000th of an 

 inch. The direction and distri- 

 bution of these tubes are ex- 

 tremely various in different ge- 

 nera : in Anomia ephippium they 

 are scantily distributed in the in- 

 ternal nacreous lamina, whilst in 

 the yellow outer layer they are 

 very abundant, forming an irre- 

 gular network (Fig. 257), which 

 spreads out in a plane parallel 

 to the surface. 



340. The ordinary account of the mode of growth of the shells 

 of Bivalve Mollusca, — that they are progressively enlarged by 

 the deposition of new laminfe, each of which is in contact with 

 the internal surface of the preceding, and extends beyond it, — 

 does not express the whole truth ; for it takes no account of the 

 fact that most shells are composed of two layers of very different 

 texture, and does not specify whether both these layers are thus 

 formed by the entire surface of the mantle whenever the shell 

 has to be extended, or whether only one is produced. An exami- 

 nation of Fig. 258 will clearly show the mode in which the 



Talmlar Shell-sti'ucture of Jnonua. 



Fig. 258. 



Vertical section of the lip of one of the valves of the shell of Uin'i>: — a, 6, c, successive formations 

 of the outer layer, a' h' c', the same of the inner layer. 



operation is effected. This figure represents a section of one of the 

 valves of Unio occidens, taken perpendicularly to its surface, and 

 passing from the margin or lip (at the left hand of the figure) 

 towards the hinge (which would be at some distance beyond the 

 right). This section brings into view the two substances of 

 which the shell is composed ; traversing the outer or prismatic 

 layer in the direction of the length of its cells, and passing 

 through the nacreous lining, in such a manner as to bring into 

 view its numerous laminae, separated by the lines a a', b b', c c', 



