BIVALVES AND CUTTLES. 



279 



). Left valve of shell of Cyprina islandica, to 

 show markings of interior, b, beak of shell ; t, one 

 of teeth ; o, anterior adductor muscle ; p, posterior 

 adductor ; I, ligament ; m, mantle-line. 



As an example of one of the simplest Eulamellibranchs 

 we may take Cyprina islandica (see Fig. 79), which is often 

 very abundant in the living condition after storms. It has 

 practically no siphons, the mantle-folds are widely open, and 

 the pallial line is 

 simple, without 

 trace of sinus. The 

 shells are large, 

 triangularj and 

 convex; they are 

 sometimes used as 

 scoops, and .called p 

 "sugar - shells." 

 The surface of the 

 shell displays ad- 

 mirably a struc- 

 ture which we 

 have not yet ex- Fm . 

 pressly noted, and 

 that is the layer 

 called by conch- 

 ologists the epidermis. The shell of a Mollusc is made 

 of three layers, an external organic layer without lime, the 

 epidermis, a prismatic layer forming the bulk of the shell, 

 and an internal pearly layer, often absent, but sometimes 

 very well developed. In many shells the epidermis is early 

 rubbed off, exposing the prismatic layer, which is often 

 brightly coloured externally; but in Cyprina it is thick and 

 persistent, giving the shell its characteristic brown colour. 

 The shell is heavy and massive, obliquely triangular, and 

 swollen towards the beaks. It is marked by numerous fine 

 concentric lines, and is covered by the brown epidermis. 

 The teeth are well developed, there being three cardinals, 

 or central teeth, in each valve, and a single lateral. The 

 interior of the shell is smooth and chalky white. This 

 combination of characters makes the species readily recog- 

 nisable. The animal lives in muddy sand, in which it 

 burrows by means of the large foot. It is sometimes used 

 as bait. 



In the next sub-order, which includes such important 

 genera as Mactra, Tellina, and Donax, the siphons are long 



