16 



simple in the right, which fits in the double lamel- 

 lar tooth of the left valve ; ligament hard, almost 

 calcareous, corneous exteriorly and convex ; two 

 unequal muscular impressions beneath the cardinal 

 tooth, the second or inferior smaller than the 

 other ; ihefossule forms a kind of fourth muscular 

 impression between the end of the lamellar tooth 

 and its muscular impression, which is separated 

 from it. Apices or summits of the valves decor- 

 ticated, and frequently the shell also ; this has 

 been erroneously employed by conchologists as a 

 specific character, as it exists in all the species, 

 except the U.Jlava, U. viridis, and young shells. 

 This effect is accidental and secondary, and the 

 consequence of their mode of living. In opening 

 and closing the valves, the animal rubs them 

 against the sand or gravel in which it resides, 

 and thus gradually wears away the apex; if it 

 resides in mud, the parts are very slowly denuded, 

 whilst among gravel the whole surface of the 

 valves becomes worn and carious. The contour 

 of the marginal edge, is, on the contrary, quite 

 entire, and hermetically closed by a prolongation 

 of the membranous and flexible epidermis, which 

 the animal forms by an exudation from its foot. 

 No part of the shell is gaping,* except in old 

 specimens. In order to complete the general 



* In the original work, the word " brilliant" is a mis- 

 print for " baillant. TRANS. 



