OF SOUTHERN INDIA. 361 



muscular scai- is situated ouly thickened, not forming a distinct hinge plate 

 which all the Dreissenid^ possess, the present classification appears to me 

 preferable. 



JlodioUiia, Miillcr, 1851. Mil Her described (Petref. der Aach. Kreidef., 1817, 

 p. 36), as Lithodomus discrepans a species which is distinguished by a very elongated 

 narrow form, somewhat of the type of the recent Z. jyaliunla, Hani., from Panama, 

 but the large anterior muscular scar rests on a thickened shelly base and is fol- 

 lowed posteriorly and above by one or two smaller scars, probably produced by 

 a division of the pedal muscle. In pt. II of the same work (1851), p. 69, 

 Muller proposed for the species in question a new generic name, " Ilodloliiia," 

 calling it 31. Bosqiieti ; the change of the specific name cannot, of course, be 

 accepted, but there is also no reason for establishing a new genus. I have examined 

 the hinge of both valves, and find that there is a strong hinge-tooth in each super' 

 seded by the usual ligamental groove, which is nearly internal. In my specimen 

 there is only one pedal impression behind the muscular scar. 



4 mppopodhim, SoAv., 1821, (Min. Con., Ill, p. 91, pi. 250). Shell ovate, 

 elongated, tumid, of solid structure, Avith sub-anterior, incurved beaks and an 

 excavated lunula ; hinge in the right valve with an elongated, almost horizontally 

 placed tooth and a pit below it, into which fits the tooth of the other valve ; 

 ligament external, long, supported by strong fulcra ; muscular scars well marked, 

 the anterior much smaller than the posterior ; pallial impression entire ; type, 

 H. ponderosnm, Sow., from the Lias. Several new species are briefly character- 

 ized by d'Orbigny in his Prodrome from Jurassic deposits. 



5. Frasina, Desh., 1863, (Mollusques de I'ile Reunion, (ext.), p. 25; Journ. 

 de Conch., 1863, vol. Ill, p. 396). Shell oblong, solid, moderately inflated, with 

 sub-anterior incurved beaks, anterior end somewhat narrowly produced, lunula 

 deep ; hinge with one strong tooth in each valve ; ligament external, long, placed in 

 a groove resting on distinct fulcra ; muscular scars two, one smaller sub-median, 

 the other larger close to former and almost united to it, being sub-central and near 

 the ventral side ; pallial line indistinct ; type, P. Burbonica, recent from Bourbon ; 

 the valves are smaragd-green when fresh and of small size. 



I have already noticed the extreme similarity of this single recent species to 

 the fossil Elppopodium, and if the position of the posterior muscular scar in the 

 latter genus should prove to be similar to that of Frasina, there would barely 

 remain sufficient ground for separating the two. 



6. Julia, Gould, 1862, (Proc. Post. Soc, VIII, p. 280). Quite similar to 

 Frasina in the form of the shell and in the teeth of the hinge, but the muscular 

 scar is said to be single, sub-central, and tripartite; the type is J. exquisita, Gould, 

 from the Sandwich Islands, apparently differing from Fras. Burbonica merely by a 

 fine radiating striation. Semper (Journ. de Conch., 1865, V, 296,) suo-o-ests that 

 this genus may be identical with the former, and this seems very probable, for it 

 is most difiicult to trace even in quite fresh specimens of Frasina the exact form 

 of the muscular scars. 



