OP SOUTHERN INDIA. 369 



obsolete, they are actually not to be distinguished. Species of Modiolaria are 

 represented in cainozoic as well as in mesozoic rocks. 



9. Arcoperna, Con., 1865, (Am. Journ. Conch., I, pp. 10 and 140). Oval or 

 oblong, thin, moderately inflated, with terminal, or very nearly terminal, incurved 

 beaks, surface finely radiately striated, somewhat stronger anteriorly, producing a 

 distinct crenulation at the margin ; hinge edentulous, ligament thin, long, situated 

 in a fine marginal furrow of the internal side ; muscular scars distinct, the anterior 

 slightly smaller than the posterior, pallial impression simple; type, A.Jilosa, Con., 

 from eocene rocks of Mississippi. Conrad says that Deshayes' Mod, radiolata, 

 (Paris, foss., 2nd edit., vol. ii, p. 22,) is cogeneric with the above species. Its relation 

 to the short, oval species of Modiola is very great, but the well marked and compara- 

 tively large anterior muscular scar, internal ligament, and fine radiating striation 

 may serve as distinction. In general character Arcoperna strongly recalls the type 

 of shell noticed as FhaseoUcama, It is not improbable that the recent Lith. cmnct' 

 mominus, Chem., is a recent representant of Arcoperfia, while, on the other hand, 

 cretaceous species, like Mytilus pileopsis, d'Orb., and a few others, are equally 

 correctly referable to it, as to Crenella, or to any of the allied genera. 



10. Marina, H. and A. Adams, 1857. Oblong, sub-equilateral, covered with 

 brown cuticule, smooth, pearly internally ; hinge edentulous, ligament interna], 

 linear, supported by a distinct rib; muscular scars distinct, wide apart; pallial 

 impressions entire ; tji^e, M. pelagica, 'Forh,, a recent species found on floating 

 masses off the Cape of Good Hope. The animal is said to be byssiferous. The 

 shell differs from Arcoperna merely by the smooth surface and more centrally 

 placed beaks. 



c. Siih-famihj—MYTIUNM. 

 Shell elongated, with sub-terminal or terminal and pointed beaks, hinge tooth- 

 less, anterior muscular scar small, marginal, posterior scar large and elongated, 

 pallial line entire. 



11. Lithodonms,^ Cuv., 1817. Shell sub-cylindrical, inflated, moderately 

 solid, very inequilateral, the beaks being almost anterior and incurved, very close 

 together ; surface smooth or often finely corrugated ; hinge edentulous, ligament 

 marginal, thin, sub-internal; muscular scars unequal, faint; type, Lith, lithophaga, 

 Linn., from the Mediterranean. 



Young Zlthodomi are stated by Cuvier to attach themselves by abyssus, which 

 subsequently, when the animals have excavated a hole, becomes obsolete. This 

 must apply either to some (perhaps not typical) species only, or to the strictly 

 embryonic state, for I have repeatedly found young specimens of 2 m. m. length, 

 (belonging to a species which grows 80 m. m. long,) perfectly enclosed in their 

 cavity. I don't think that old Lithodomi would at all be able to bore a hole. 



* I think that it is preferable to retain Cuvier 's name before any other. Bolt en's name Lithophaga (also often 

 used as a family name by other authors) appears to have no claim to priority, but M. von MUhlf eld's Lithophagus 

 would have this, were it not a mere repetition of the specific appellation Mytilus litJio'phagus, Linn., this being the 

 type of Lithodomus. 



