LUCINA. 61 



rendering its description and determination a task of 

 more than ordinary difficulty, has led to the separation of 

 this shell into three species, a division which the close 

 examination of a long suite of examples (including the 

 types of the whole) compels us to regard as unneces- 

 sary. 



The shape is more or less obliquely orbicular subcordi- 

 form, and is usually, but not invariably, longer than broad. 

 The valves, which are thin and fragile, vary much in 

 profundity, but are always more or less swollen at the um- 

 bones, become, if anything, rather more compressed behind 

 than in front, and diminish rapidly in convexity towards the 

 ventral margin : this latter is greatly arcuated anteriorly 

 and obliquely, and rather suddenly ascends in a straigliter 

 line on the posterior side of the shell. The surface, when 

 cleared of the ferruginous earth, which is often disposed in 

 concentric waves, is quite smooth, and, in fine and recent 

 examples, even moderately glossy white and semitranspa- 

 rent, but is more usually met with dull, opaque, and partly 

 eroded. The sides are nearly equal : the front one is ob- 

 tusely and rather broadly rounded at its extremity ; the 

 hinder is bluntly angulated below. The posterior dorsal 

 edge is greatly the more elevated, and runs uninterrupted- 

 ly with a more or less arcuated sweep to the lower margin ; 

 the front dorsal edge is very short and little sloping ; it 

 bends inwards imder the beaks, which are acute and in- 

 clined. The umbones, which lean greatly forward, are 

 prominent, occasionally projecting very considerably. There 

 are no dorsal impressions, but the hinder area is perhaps a 

 little flattened. There is neither fold nor umbonal ridge, 

 and the ligament is not prominent, but is semi-internal, the 

 hinge-margin being slightly excavated for its reception. 

 The interior is glossy, and either white or occasionally 



