OP SOUTHERN INDIA. 333 



to that of the cretaceous alone, and certainly it is not larger, while that of the 

 present epoch again seems to have somewhat decreased. I would estimate the 

 number of species of the arcing in the whole of the palaeozoic period to be 

 about 50 ; in the Trias about 40 ; in the Jura barely 80 ; from the cretaceous 240 

 are now on record, from the eocene about 120 are known, from the miocene, 

 oligocene and pliocene about 100, and recent there are about 180 described. The 

 general conclusions based upon these figures can of course be accepted only as 

 approximately correct, for there are not nearly so many additions to be expected 

 in the recent as in the fossil fauna. 



With reference to the distribution of the recent species, I hardly need to remark 

 that the tropical seas contain by far the larger number. Most of the species are 

 truly marine shells, only a few are found in brackish water, and even Scaphula, 

 which is generally quoted as a mere freshwater inhabitant, seems quite as common, 

 if not more common, in the brackish waters of the delta of the Irawadi, as where 

 it occurs in the Ganges near Monghyr, (see Blanford, Jour. As. Soc, Bengal, 

 xxxvi, pt. II, p. 70). 



a. 8ul-family,----AXINMINM. 



1. NtiGunella, d'Orb., 1850, (Prodrome de paleont., ii, p. 389).* Shell sub- 

 orbicular, slightly inequilateral, beaks close together; hinge line curved with 

 numerous cross pliciform teeth, interrupted under the beaks by an oblique simply 

 granular pit, this last appearing to be destined for the ligament, which must at least 

 partially be internal; type, N, Nystii fStalagmium Nystii, Galeotti), from tertiary 

 beds of Belgium. This appears to be quite distinct from Zimopsis, but I am not 

 certain of the distinction from Myoparo or Stalagmkim, Ohenu (Man., ii, p. 181,) 

 quotes a N, aviculoides of d' Archiac, which is either a peculiar form of a CucuUcea, 

 or else a distinct genus of the Arcid^. Lea (Cont., p. 73,) does not say in his 

 characteristic of Myoparo (1833) whether the type species, M. costatus, has an in- 

 ternal cartilage pit, but the figure seems to indicate one, quite similar to that of 

 N, Nystii, and this makes the identification problematical. 



2. Trigofiocoelia, Nyst and Galeotti, 1835. Shell sub-trigonal, inequilateral, 

 or nearly so, posterior declivity carinated and usually pointed at the end ; hinge 

 line small, curved, with numerous cross-teeth ; no separate hinge area above it, 

 but a small cartilage pit is situated immediately below the beak ; type, T, incequi- 

 lateralis,^ d'Orb. Deshayes, I think, properly reserves Nyst's name for the 

 triangular forms, which on account of the want of a special hinge area mostly 

 approach the Nuculidje. 



3. Limopsis, Sassi, 1827, CFectunculina, d'Orbigny). Shell sub-orbicular, 

 sub-equilateral ; hinge line curved and provided with a series of cross-teeth, a small 

 hinge area below the beaks provided with a triangular pit in the middle^ sometimes 



* D'Orbigny gives 1847 as the date of publication of tbe name, and refers to his Cours de paleontologie, which 

 (Vol. II) bears the date of 1852, while the Prod, was published in 1850. Chenu writes Nuculella, but this name was 

 not used by d'Orbigny in any of the publications noted above. 



t Not inceqtdvalvis, as quoted by Chenu. 



