OP SOUTHEEN INDIA. 35 



1 could add here the fossil genus Quemtedtia, but as it does not seem to possess 

 internally a cartilage, it had better be removed to the Saxicavid^, 



2. SpJienia, Turton, 1822. Shell oblong, gaping posteriorly, covered with an 

 epidermis, which extends over the united siphons ; right valve larger than the left. 

 Hinge with two (one above the other) small teeth in the right, and one in the left, 

 or with one tooth in the right valve and a corresponding pit in the left one ; or the 

 teeth are in both valves obsolete ; cartilage internal, situated in a separate oblique 

 pit in each valve; pallial sinus moderate; muscular impressions comparatively 

 large. 



H. and A. Adams correctly remark that this genus ought to be restricted to 

 the oblong or sub-quadrangular species of the Sph. Binghami type ; several other 

 forms described as Sphenia partially belong to Corbulomya, Corhula, and allied 

 genera. The prolonged siphons and the short digitiform byssiferous foot fairly 

 distinguish the animals of this genus from those of Corbula. The hinge is occa- 

 sionally very similar in both, but seems to vary a great deal, as noted above. I 

 observed those variations of the hinge-teeth in two species found living about 

 Calcutta, both allied to Binghami. The development of the teeth does not always 

 depend upon the age, some young specimens having them perfect and others not, 

 but, as a rule, the hinge-teeth become obsolete only in adult shells, thus evidently 

 indicating a relation to some of the Gastrocr^nid^. There is a distinct process 

 in each valve (though larger in the left one) for the reception of the cartilage^ 

 while in Corbula such a distinct process does not exist in the right valve, the 

 cartilage lying there in a deep pit. Besides this Sphenia occasionally possesses a 

 trace of an external ligament, placed at the upper end of the cartilage-pit, just 

 behind, or rather below, the beaks ; of this ligament I could find no distinct trace 

 in several species of Corbula examined for that purpose. 



3. Corbula, Brug., 1792. Shell inequivalve, the left valve being the smaller 

 one ; covered with a more or less rough epidermis ; hinge with one prominent tooth 

 in the right valve, in front of a cartilage-pit, corresponding to a strong cartilage 

 process in the left valve ; pallial sinus very small or not marked at all. I am not 

 acquainted with any fresh water species of true Corbula, but there certainly are 

 brackish forms, which cannot be generically separated from the typical marine 

 species of this genus, except that their shells are remarkably thin, resembling those 

 of Nemra. 



A very peculiar tertiary form of a Corbula (n. sp.), in external appearance not 

 unlike an -^^ara, is described by Helmerson in the Bull, phys.-math. de I'acad., 

 St. Petersburg, (1845, vol. iv, p. 7). It has a broad tooth in each valve and a 

 groove next to it for the reception of the tooth of the other valve. In the 

 left valve there are two partially obsolete vertical ribs below the umbo. It is 

 probable that this form to which, as far as I know, no specific name is yet applied, 

 will have to be made the type of a new genus, but this cannot be done without 

 previous careful comparison of the typical specimen. 



4. Biicharis, Becluz, 1850, Journal de Conch., i, p. 167, {? = Basterotia, 

 Mayer, 1859, in Homes' Poss. Moll, des Tert.-beckens von Wien, vol. ii, p. 40). 



