TRIBE PHOLADARTA. 7 



rounded oif subtruncation, smooth ; posterior ventral margin with a 

 very large gape : accessory valve solitary and posterior : lanceolate 

 and acuminated posteriorly. 3.. Ifn. — W. Columbia. Very like 

 Parva. 



* * Valves divided h/j an indented radiating groove. 



P. Crispata.* Lin. 1111.— J>. p. 40.— Zam. '] .—Ch.f. 872, 

 3, A.-Don. t. Q2.— Wood. G. C. t. 15. /. 4, 5.— Mont. p. 23.— 

 Turt. B. p. 6. — W. t. 2.f. 5. Oval, gibbous, whitish, rounded 

 at one end, and forming a short beak at the other ; a longitudinal 

 furrow in the middle, on the posterior side of which the shell is covered 

 with muricated striae, on the anterior transversely wrinkled : gape 

 wide. 11 . .2\. — N. and W. Europe. 



P. Clavata.* Lam. 9.— P.Pusila. Lin. 1111.— Z>. p. 33.— 

 Ch.f. 867,8, 9.— P. Striata. Lin. 1111.— TF. t. 2./. 7 .— Wood. 

 G. 'C. t. 16. f. 1, 2, 3,4, 8.— Don. t. 116.— Reeve, t. 24./. 2.— 

 Sow. G.f. 2. — D. p. 37- Oblong, rounded, white, and of a re- 

 markably conoid shape ; the broader end strongly reticulated by 

 arched stria3, the other parts except a smooth triangular space at 

 the posterior ventral margin faintly striated : the shield which 

 covers the hinge heart-shaped, and beneath it a narrow plate joined 

 to the membrane which connects the valves ; a narrow ventral plate 

 and membrane which seems to prevent the shell from opening except 

 at the ends, i . . 1. — America, tj'c. 



P. Ovum. Gray in W. S. f. 4. Sub -cuneiform, posteriorly 

 inflated, rounded and somewhat gaping, anteriorly attenuated : 

 valves divided by a longitudinal groove, anterior to which the shell 

 is nearly smooth ; the posterior dorsal portion with minute raised 

 oblique wavy wrinkles crossing the more or less distinct radiating 

 riblets, posterior ventral portion with distant divaricating striae ; 

 hinge margin raised, reflected : accessory valve sub-rotund, dorsal 

 and posterior : teeth thin and flattened . . 1;^. — The young without 

 the posterior ventral portion. The sulcus forms a distinct tooth 

 at the interior ventral edge. 



P. Falcata. Wood. G. C. t. 16. /. 5, 6, 7.—D. p. 3<}.— W. 

 t. 2. /. 8. Suboval, ventral margin posteriorly much incurved, 

 divided by a longitudinal groove, the posterior compartment being 

 very tumid, short, and rough with sub-dentated striae, the anterior 

 compressed, with simple transverse striae : whitish, the gape wide, 

 the hinge with a forked tooth. -|. — Shape of Crispata. 



P. Calva. Gray. Z. P. 1834. Ovate, posteriorly retuse, 

 anteriorly sub-acuminated, gaping ; each valve divided into 3 sur- 

 faces, the posterior one roughish, the middle one with a horny 

 epidermis and concentrically striated, the anterior one with horny, 

 imbricated, smooth scales, which gradually become less, and are 

 anteriorly rounded : the posterior ventral space smooth and closed : 



