66 SYNOPSIS OF THE CHARACTERS OF THE 



DOLABRA ORBICULARIS. M'Coy. (PI. XI. fig. 13). 



Sp. Ch. Orbicular, depressed, smooth ; length and width equal ; beaks central ; posterior slope compressed, 



posterior ridge obtuse. 



This species is remarkable for its nearly orbicular form. In some of the casts the impressions of the car- 

 dinal teeth are visible. Length five and a half lines, width the same. 



DOLABRA RECTANGULARIS. M'Coy. (PI. XI. fig. 10). 



Sp, Ch. — Subquadrate, length and width nearly equal, convex; beaks nearly central, very large, promi- 

 nent ; anterior side obtusely rounded, front margin convex ; posterior side short, flat, rectangular, abruptly trun- 

 cated, separated from the body of the shell by a very prominent diagonal ridge from the beak ; surface 

 smooth. 



This shell much resembles the CucullcBa anc/usta. Sow., in general form, but is much more depressed, 

 and is moreover distinguished by its greatly larger beaks, and prominent posterior diagonal ridge ; the straight 

 rectangular posterior side distinguishes it from every species of Cypricardia with which I am acquainted. 

 Leno-th six lines, width seven lines. 



DOLABKA SECURIFORMIS. M'Coy. (PL XL fig. 15). 



Sp. Ch. — Rhomboidal, convex, securiform; anterior side very short, broad, rounded, front margin convex ; 

 a slight sinus at the posterior angle ; posterior side produced, flattened, obliquely truncated, narrow at the end, 

 separated from the body of the shell by an obtusely rounded ridge ; hinge-line three-fourths the width of the 

 shell ; surface very finely striated transversely. Length ten lines, width one inch two lines. 



DoLABRA TRAPEZIUM. SoW. SP. 



CucuUffia trapezium. Soie. Geol. Trans CucufliBa trapezium. Phil. Pal. Fos. 



The few specimens which have occurred of this shell are too imperfect for description ; the species re- 

 sembles the D.Hardingii, but is much longer in proportion to the width, and has the posterior end broader and 

 more truncate. 



DoLABRA UNILATERALIS. SoW. SP. 



Cucullaea unilateralis. Sow. Geol. Trans. 



Sp. Ch. — Obliquely trigonal ; beaks large, tumid, nearer to the anterior than the posterior side ; hinge-line 

 short ; margin one regular curve from the anterior end to the posterior angle, which is much lengthened, and 

 having a strong ridge from the beak ; posterior end very obliquely truncated. 



Leptodomus. M' Coy. 



Gen. Ch. — Transversely oblong, equi valve, inequilateral; shell very thin, inflated, transversely sulcated ; 

 beaks large, tumid, obliquely incurved ; hinge-line moderate, nearly the length of the shell ; anterior side 

 short, rounded ; posterior side subtruncate ; muscular impressions two, distant, lunate. 



This genus is proposed to include a niimber of fossil shells hitherto placed witli Corlmla, although differing 

 from that genus in almost every essential point ; in the first place, all the CorhulcB have one valve much 

 ■ smaller than the other, particularly at the beaks, while both the valves and beaks of the present shells are per- 

 fectly equal ; the beaks of Curhula are nearly central and straight, they are close to the anterior end, and 



