MONTACUTA. 73 



a thick coating of rust-coloured earth, from which circum- 

 stance the shell has derived its specific epithet. The ven- 

 tral edge is but slightly convex, but curves upward at both 

 extremities. The hinder side is produced ; its termination 

 is usually well rounded, and a little tapering, but occasion- 

 ally is subangulated above ; its dorsal edge (except in the 

 elongated typical variety, where all the edges are compa- 

 ratively rectilinear) is more or less convex, or even arched 

 towards its termination, but straightish or even subretuse 

 near the umbones ; its declination is extremely trifling. 

 The shorter anterior side exhibits a considerable difference 

 in its aspect, being occasionally much abbreviated and 

 rounded, and rather broad at its extremity, at other times 

 slightly more elongated, and from the greater declination 

 of the dorsal edge is narrowed and subangulated at its 

 lower extremity. The beaks are small, acute, and directly 

 inflected, not leaning to either side ; they are distinct, but 

 by no means prominent. The inside is white, the margin 

 plain and acute, with often a slight vestige of a subcentral 

 retusion. The hinge-margin, which is interrupted by a 

 trigonal incision beneath the beaks, is furnished with an 

 erect narrow simple and almost vertical tooth behind the 

 notch, and a more oblique and rather less immediately 

 approximate acute conical denticle before it, which latter 

 projects downward and inwards, and forms apparently the 

 terminal wall to an appressed and anterior cartilage-pit. 



In the right valve, where the apical perforation for the 

 ligament is most evident, the posterior tooth is less de- 

 veloped, hardly amounting in some individuals to more 

 than an elevation of the cardinal edge. 



There are two most distinct forms of this shell ; the 

 rarer (which is that figured by Montagu, who, however, 

 possessed both varieties, and regarded them as specifically 



VOL. II. li 



