OF SOUTHERN INDIA. 339 



sub-equal transverse or slightly oblique teeth ; surface smooth, or finely radiately 

 striated; type, /. clecussaia, Munst., 1. cit., pi. iv, fig. 14. The species of Isoarca 

 are from Jurassic and cretaceous dejiosits. Their peculiar, mostly decussated sur- 

 face greatly resembles that of many Barhatice, but the structure of the sliell itself 

 appears to be somewhat different from that of other arcing, being more solid and 

 pearlaceous. Probably Isoarca will better be classed in the family Nucvlibje near 

 the problematic genus Ctenodonta of Salter. 



14. Argina, Gray, 1812. Shell sub-cordate, inflated, sub-equivalve, inequi- 

 lateral, beaks somewhat anterior, radiately ribbed ; ligamental hinge area very 

 narrow, hinge line slightly elongated and curved, divided by an indentation under 

 the beaks into unequal portions, both being proidded with numerous sub-equal 

 cross denticulations, those on the anterior portion being fewer and irregularly 

 crowded; type, A. pexata. The species are few, and from the American seas. 



15. Senilia, Gray, 1847. Shell sub-cordate, tumid, inequilateral, posteriorly 

 somewhat produced and attenuated, I'adiately costated, covered with an olivaceous 

 epidermis ; ligamental hinge area broad, with angularly bent strise ; hinge line 

 nearly straight, with numerous rather large sub-equal cross-teeth, internal edge 

 below the umbones emarginated; type, S, senilis, Linn., from brackish waters 

 of West Africa. It is the only species as yet known, and may be considered as 

 the brackish water representant of Aiiomalocardia. 



16. Aiiomalocardia, Klein, 1753, (Teutamen Ostr., p. 141, Aiiadara, Gray, 

 1847). Shell sub-quadrangular or trapezoid, sub-equivalve, sub-eqmlateral, solid, 

 radiately ribbed ; ligamental hinge area large, angularly striated ; hinge line 

 straight,with numerous sub-equal cross-teeth, somewhat oblique towards the ends; 

 type, A. antiquata, Linn. 



16a. Scapharca, Gray, 1847. This has been proposed for a number of 

 inequivalve species, like Area incequivalvis, Brug., but this character is not 

 exclusively common to the species referred to that genus by H. and A.Adams; 

 several species o£ Aiiomalocardia, as, for instance, A. ^rrrtKo^a, Linn,, and others 

 may often be seen inequivalve. However, though the form of these shells is 

 quite the same as that of Anomalocardia, those of Scapharca are always of a 

 thinner structure, with internally hollowed out ribs, somewhat inequilateral, and 

 the ligamental area is mostly narrow. Only on this account Scapharca may be 

 retained as a sub-genus of Anomalocardia, though in determining fossil species 

 the division is of no great importance. 



Fossil species of Anomalocardia occur from the Trias upwards, but until in 

 the miocene time they are nowhere very numerous. 



166. Nemoarca, Conrad, 1869? (Am. Journ. Conch., V, p. 97). Shell 

 elongately trapezoidal, sub -equilateral, tumid, radiately ribbed ; ligamental area 

 narrow; hinge line straight, with small transverse teeth; type, N. cretacea, Con. 

 (1. cit., pi. 9, fig. 21). I do not know the distinction of this newly proposed 

 genus fi'om the elongated forms of Scapharca, as, for instance, Scapharca for-^ 

 mosa, Sow. 



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