ANATINACEA. 75 1 



equivalve, but very inequilateral, possessing an external ligament, and a 

 strong external oblique posterior ridge, the pallial line being entire " (R. 

 Etheridge, jun.) The Carboniferous genus Anthracomya, in which there 

 is a thin concentrically-striated oblong shell, with anterior beaks and an 

 external ligament, may perhaps also find a place here. The Devonian 

 genus Palaanatina (fig. 636, a) more probably belongs to the AnatinidcE, 

 since the shell is inequivalve (the left valve being larger than the right), 

 and there are indications of the presence of an internal ligament ; but the 

 form of the pallial line has not been clearly ascertained. The Devonian 

 genus Cimitaria also occupies an uncertain position ; and there are 

 various other Palaeozoic types which may possibly be referable to this 

 family, but which cannot be discussed here. 



Family 5. Pr^ecardiid^. — This family has been constituted by 

 Hoernes for the reception of a number of Silurian Bivalves, of which 

 the genus Prcecardium is the type. The forms here in question 

 have thin equivalve or inequivalve shells, which have a marked 

 general resemblance to the Cockles, but which are without the 

 characteristic teeth of the Cardiidce — the hinge being, in fact, 

 edentulous, or furnished with transverse folds. The adductor 

 impressions and pallial line in Pracardium and its allies are un- 

 known ; but Fischer suggests that the genus Cardiola may possibly 

 be found to belong to this singular group. 



Family 6. Pholadomyid^e. — In this family the mantle-lobes are 

 united ; and there are very long siphons, conjoined along their 

 entire length. The foot is small, and a single gill is present on 

 each side. The shell is equivalve, inequilateral, thin, convex, 

 adorned with radiating ribs, pearly internally, but not granulated 

 externally. The hinge is toothless ; the ligament is external ; and 

 the pallial line is sinuated. This family has close relationships 

 with the Arcomyida, and also with the Anatinidce, and the only 

 genus included in it by Fischer is Pholadomya itself, which is 

 extremely abundant in the Jurassic, Cretaceous, and Tertiary rocks, 

 but is only known at the present day to be represented by two or 

 three species, all of which inhabit great depths in the sea. 



In Pholadomya (fig. 637) the shell is equivalve, oblong, and 

 gaping posteriorly, thin, ven- 

 tricose, and adorned with 

 radiating ribs on the sides. 

 The anterior side of the shell 

 is short and rounded, and the 

 beaks are prominent. The hinge ^ISBSS^ ^ffl^ 5 ' 



is tOOthleSS, and the Cardinal Fig. t^.—Fholadovtya cequivalvis. Chalk. 



margin behind the beaks is 



often folded in so as to give rise to an elongated false area or 



escutcheon. 



Family 7. Clavagellid^:. — In this family the mantle-lobes are 



