CERATISOLEN. 257 



straight, but the former is slightly the more sloping, aiul 

 the latter exhibits a very trifling degree of convexity. 

 Neither the beaks nor the umbones are at all conspicuous ; 

 the blackish ligament, which is large and somewhat trian- 

 gular, expanding greatly behind, does not project above 

 the dorsal line, an incision being made in the valves for its 

 reception ; its hinder termination is about the centre of the 

 hinge-margin. The posterior side is nearly double the 

 length of the anterior one, and is rather broad and very 

 obtusely rounded at its termination ; the upper side of the 

 attenuated front extremity projects in a trifling degree 

 beyond the lower one, so that the anterior outline is 

 obliquely rounded below, and obsoletely subangulated 

 above. There is no appearance of any umbonal ridge, nor 

 the slightest inflection of the margin on either side of the 

 beaks. 



The inside is usually whitish, but is sometimes stained 

 with orange upon the subumbonal region. The hinge is 

 very complicated ; the hinge-margin is strengthened near 

 the beaks, in both valves, by a very short obliquely radiat- 

 ing rib-like callosity ; in the left valve there is a single 

 upright greatly compressed primary tooth, and an ex- 

 tremely approximate bifid sublateral one juts out be- 

 yond the hinge-margin, which ceases near the commence- 

 ment of the ligament. In the right valve are three teeth, 

 the posterior sublateral a mere denticle, and the cen- 

 tral broadly based and bifurcated ; which two constitute, 

 with the curved posteriorly inclined primary anterior one, 

 a double pair of nipper-like receptacles for the teeth of the 

 opposite valve. 



The animal is of a linear oblong shape ; its mantle, of a 

 yellowish- white colour, is closed in front, and open ante- 

 riorly for the passage of the foot. Its edges at the open- 



A'OL. I. L L 



