INVERTEBRATE PALEONTOLOGY. 213 



PHOLADOMYIDiE. 

 Genus PHOLADOMYA, Sowerby. 



Stjnon.—Pholadomija, G. B. Sowerby (1823), Genera of Shells, fasc. 19— Latreille (1825), Fain. Nat.— 

 Defranco (1826), Diet. Sci. Nat., tab. xxxix, 535.— J. Sowerby (1827), Man. Conch., 

 tab. 545.— Deshayes (1830), Encyc. Mc"th., Ill, tab. 1832 ; ib., 756, and (1835) in Lam., 

 2c ed., VI, 63.— F. Roenier (1836), Verst. Ool., 126.— Broun (1837), Leth., 384.- 

 Agassiz (1842), Etud. Crit. Moul. Foss.,2e liv., 37.— Hanley (1842), lllust.Cat., App., 18; 

 and of numerous other authors. 



Pholadomyaa, Fleming 1 1828), Hist. Brit. An., 408 and 4'2~. 



Procardia, Meek (1871), Proceed. Acad. Nat. Sci. Philad., XIV, 184 (subgenus). 



Ehjm.— rholas ; Mya. 

 Type. — P. Candida, Sowerby. 



Shell equivalve,* inequilateral, varying according to the species and 

 sections, from transversely-oval or oblong, to rhombic, subtrigonal, trape- 

 zoidal, &c, generally ventricose anteriorly, pearly within ; posterior extremity 

 always more or less gaping; anterior closed or somewhat gaping; beaks 

 varying greatly in prominence, convexity, and obliquity ; hinge with an 

 obscure, transversely slightly elongated cardinal tooth, and a small pit in each 

 valve; cardinal margin often inflected, so as to form a kind of false area 

 behind the beaks ; ligament small and external ; muscular and pallial impres- 

 sions most usually obscurely defined, the latter provided with a moderately 

 large sinus. Surface ornamented by radiating, often nodular or tubercular 

 costal, crossed by concentric striae, or furrows. 



The species of this extensive genus present very great diversities of 

 form, ornamentation, and general appearance; yet, as remarked by Professor 

 Agassiz, it seems to be very difficult, at least in the present state of our 

 knowledge of these shells, to divide them into well-defined sections or sub- 

 genera. This difficulty arises, in part, from the gradual blending together, as 

 it were, of widely different forms, and also, in part, from the great acci- 

 dental distortions to winch, owing to the thinness, and perhaps to some 

 extent flexible nature of these shells, they have often been subjected, as we 

 find them in the rocks. 



Professor Agassiz, in his excellent monograph of the genus, separated 

 the species then known into two primary divisions, distinguished by the first 

 having a more or less defined false cardinal area, and the second by being 

 nearly or quite destitute of the same. 



* As noticed by Professor Agassiz, the costa: of the left valve arc often more numerous and distinct 

 than those of the right; thus indicating a very slight tendency to inequality of the valves, in some 



