454 CYPRINID^. 



narrower than their interstices, and are rarely if ever much 

 depressed or at all broad. They typically cover the entire 

 surface down to the ventral margin, excepting that they 

 usually vanish (but not in the young), beyond the ordinary 

 site of the obsolete umbonal ridge towards its extreme 

 termination, but are generally visible for about half-way 

 down that posterior area. Besides these ribs which, 

 moreover, continue quite down to the ventral margin, 

 which is always more or less convex, and at times (when 

 the declination of the dorsal edges is more than ordinarily 

 strong), is even arcuated, there is no sculpture. The 

 sides are unequal, but for the genus not particularly so, the 

 beaks, which are acute, prominent, and a little inclined, 

 being situated about one-fifth, at most, before the middle of 

 the dorsal edge. This latter is concave and greatly sloping 

 in front from the deep excavation of the very large lanceo- 

 late or ovato-lanceolate lunule, which, as well as the 

 linear lozenge running nearly the entire length of the 

 hinder dorsal margin, is perfectly smooth ; the hinder edge 

 is by no means so sloping as the front one, but varies from 

 slightly to moderately declining. The ligament is small 

 and yellowish, and is seated in the lozenge of whose length 

 it only occupies about two-fifths. The anterior extremity 

 is well rounded ; the hinder termination is more or less 

 bluntly biangulated, and in the young very broadly so. The 

 interior is usually white, a small northern variety is, how- 

 ever, of a reddish liver colour. The edges are adorned ty- 

 pically in the adult with closely-set bead-like elevations, ex- 

 cepting upon the hinge-margin, which is moderately broad, 

 and provided in the right valve, with a solid central simple 

 triangular tooth, which shelves inwards, the highest point 

 not being, as in most shells, at its lower end, but mid- 

 way from the base of the margin ; in the other valve a 



