OP SOUTHERN INDIA. ' 275 



ciently known, and, therefore, tlie classification somewliat doubtful. In external 

 form, thickness of the shell, and the rovigh appearance of the anterior side, the 

 shells greatly recall some species of Astarte and of Crassutella. 



3. llegalodon, Sow., 1827, (Min. Conch., vol. vi, p. 131). Shell ovate or 

 sub-triangular, moderately inflated, with prominent incurved beaks, of solid struc- 

 ture ; hinge with two generally more or less distinctly bifid cardinal teeth, the 

 anterior being smaller than the posterior, which is considerably elongated and 

 often of unequal size in the two valves ; a thin posterior marginal lateral tooth is 

 often present ; anterior muscular impression small, deep, very close to the hinge- 

 teeth, sometimes with a superimposed small impression, posterior muscular impres- 

 sion larger, elongated, occasionally on a raised lamina ; pallial line entire, ligament 

 strong, external, supported by thickened fulcra. 



Giimbel, in his paper on the " Dachsteinbivalve, &c.," (Sitzb. Akad., Wien, 

 1862, vol. xlv, p. 361, &c.), proposes to. divide the genus into three sub-genera. 



3 a. Eii-megalodon, restricted to the palaeozoic species, of which M. cuoullatus, 

 Sow., is tlie type. The form of these shells is generally considerably elongated, 

 resembling Megalomiis, and sometimes longitudinally carinated ; the posterior cardi- 

 nal tooth in the right valve is smaller and widely bifid, the corresponding tooth in 

 the left valve very prominent and only grooved, the posterior lateral tooth is Avell 

 developed. 



3 h. Neo-viegalodon, as the type of which 3Ieg. triqueter, Wulfen, may be 

 considered ; the shells are sub-trigonal or cordiform, posterior cardinal teeth bifid 

 in both valves, stronger in the left than in the right one, posterior lateral tooth 

 indistinct or obsolete. All the species are triassic. 



3 c. Tacliy-megalodon ; type, II. chamcefurmis, Giimbel, also from triassic beds. 

 Posterior cardinal tooth single in the left, double and curved in the right valve, 

 posterior lateral tooth short and distinct ; anterior muscular impression posteriorly 

 and inferiorly surrounded by a raised margin. 



4. Astartila, Dana, 1849, (vide Geol. Expl. Exp., p. 088). Sliell elougately 

 sub-triangular, inequilateral, slightly inflated, moderately thickened at the beaks 

 and near the margins, thin in the middle, on the surface concentrically striated or 

 lamellated ; ligament external, very long ; muscular impressions rather large, the 

 anterior close to the hinge and with a small superimposed impression ; pallial line 

 entire; hinge unknown. Type, A. inlrepida, Dana. 



This genus is based upon a number of carboniferous species from New South 

 Wales ; they greatly resemble in external form the sub-genus Caryatis of Cytherea, 

 but the long ligament and the muscular impressions appear to be very similar to 

 those of Astarte. The examination of the hinge-teeth is required for the correct 

 determination of the family to which these shells belong. 



5. Conchodon, Stopp., 1861, (Pal. Lomb., 3"'^' ser., 16"'= livr., p. 246). Shell 

 cordate, thick, concentrically striated on the surface, with the beaks prominent and 

 incurved ; hinge area very large, composed of three sub-equal transverse jiosterior 

 cardinal teeth, the uppermost in the right valve being longest and strongly curved ; 



3 X 



