34 CEETACEOUS PELECYPODA 



have the siphons short; the shells of the other sub-family have a very large 

 cartilage process in the left valve, - except Tugonia, which has it in both, - 

 they are elongated and gaping posteriorly ; the animals have long united siphons. 

 If we compare Mya with Cryptomya {Nuttallii, for instance), the principal 

 difference to be detected between the two is the very short pallial sinus in the 

 latter. Some species of the fossil Corhulomya, and other recent ones belonging to 

 Azara, fully establish again a connection between Gorhulomya and Corbula, both in 

 form, position of the cartilage, and pallial impression. Necera inclines to the 

 Anatinid^ in the structure of its shell, but its form and the organisation of the 

 animal place it near Splienia. Foromya, especially in its fossil forms, of which only 

 the last remnants appear to have survived the great terrestrial and climatal changes^ 

 must, according to the organisation of the animal, the form of its shell and that of 

 the hinge, be placed near Corbula, though the structure of the shell and the pre- 

 sence of a rather strong external ligament have some relation to Fholadomya. Thus 

 it is at one time the majority and at another time the importance of the characters, 

 common to a number of shells, which must be made use of in classification. 



Of all the generic types Sphenia is most closely allied to the GastrochjeniDuE. 

 The animal is almost identical with that of Bocellaria, and the shell is very similar 

 to it, especially when the hinge-teeth become obsolete. It is generally indented on 

 the antero-ventral margin, indicating the gape of Bocellaria, I have besides 

 observed species of Sphenia which sometimes form posteriorly a regular prolonged tube, 

 in as much as the epidermis, covering the siphons, becomes very much hardened. 

 Its burrowing habit also indicates the relation just mentioned. 



The 5th part of the 4th Volume, 1868, of the American Journal of Conchology 

 (Appendix p. 62, &c.), contains a catalogue of the recent species of the Myidjs and 

 CouBJJLiBM by G. W. Tryon, Junr. The former contains 10 species and the latter 

 85, excluding NecBva and Foromya; but, even neglecting these two last named 

 genera, the list does not appear to be as complete as it would seem desirable. 



I shall again, as on the previous occasions, first give a short review of the 

 recent and fossil genera, referable to the family, then note the species represented 

 in cretaceous rocks, and at last give descriptions of the species occurring in the 

 South Indian cretaceous deposits ; the latter are characterized by three genera only, 

 Corbula^ Foromya^ and Necera, 



a. Buh-family,—CORB VLINM. 



The genera, as far as known, appear to arrange themselves naturally as follows : 

 1. mmella, H. Adams, 1860, (Proc. Zool. Soc. for 1860, p. 369). Shell thin, 

 with the left valve larger than the right one, not gaping ; hinge of the right valve 

 with an indistinct tooth fitting into a pit in the left valve ; cartilage internal, lying 

 in both valves in an almost horizontally extending process ; an external ligament 

 is besides present; pallial sinus scarcely noticeable. This genus is based upon 

 a fluviatile species, S. fluviatilis, from the Maranon river. 



