212 SYNOPTIC COLLECTION. 



extended from the shell; at one end is the siphon and at 

 the other the foot (Nos. 417, 418). 



Donax scortum Linn. (No. 420), is triangular in shape. 

 The hinge area has two cardinal teeth on one valve and 

 one on the other that fits between the two. The two 

 marginal teeth fit into depressions. A rounded ligament 

 is external. 



Cardium edule Linn. (No. 421), is represented by shells 

 of different ages. At first the shell has shallow grooves 

 or plications, but these grow deeper and cause the shell 

 to be thicker and stronger. The hinge has both tranverse 

 and parallel teeth. The umbos are close together and the 

 ligament is external. The muscle impressions are distinct, 

 but the mantle mark is often obscure, while there is no 

 siphon impression, although the animal has a short siphon. 

 In Cardium the foot (No. 422, C. aculeatum) can be 

 suddenly bent so that the animal leaps through the water. 



The changes in color of the external horny layer are 

 finely illustrated by Cyprina islandica Lam. The young 

 (Nos. 423, 424) is light brown in color. This becomes 

 a rich shade of brown in the adult, while in the old shell 

 (No. 424, specimen at the right) it is nearly black, and so 

 brittle that it can be easily scraped off (No. 424). The 

 ligament is partly internal and the hinge has three cardi- 

 nal teeth besides the marginal teeth. The two muscle 

 impressions and the mantle impression in this shell are 

 often distinct but the short siphon makes no mark. 



Cytherea ( Venus) verrucosa Linn. (No. 425), is a large 

 plump shell when full grown" with prominent beaks, be- 

 hind which is a large external ligament. The well devel- 

 oped hinge area has three cardinal teeth. The shell is 

 porcellanous like most of the more specialized shells, with 

 both muscle, mantle, and siphon marks well shown. 

 Smith says 1 that one species of this genus (C. torresiand] 

 has the posterior end broader in the early stages than 



1 Chall. Rep., Zool ., XIII, part 35, 1885, p. 119. 



