OF SOUTIIEUN INDIA. 205 



is grown together posteriorly with the corresponding plume of the other side ; the 

 palpi are four, close together, and slender. 



I shall keep the forms of the above type under the name of cardiinm. Their 

 shells are equivalve, more or less cordate and usually inflated, sometimes moder- 

 ately elongated, rather robust, and ornamented, partially or wholly, with radiating 

 tuberculated or spiny ribs, wliieh in rare instances are reduced to mere strife. 

 There are two muscular impressions ; the inner margin of the shell is partially or 

 wholly crenulated or toothed ; the ligament is external, the pallial impression gener- 

 ally entire, truncated sometimes but rarely, with a short sinus. The hinge is 

 composed of two cardinal teeth in each valve which cross each other when the valves 

 close ; there is one anterior and one posterior lateral tooth present, rarely becoming 

 obsolete. 



The position of the cardinal teeth is one of the most characteristic features 

 in the shells of this family, and readily distinguishes even those which, like 

 LcBvieardium and others, in external form resemble species of the Cteenid^. 

 Even where one or the other of the cardinal teeth become obsolete the remaining 

 retain their crossing position. 



The brackish- and fresh- water Cardia of Eastern Europe appear, however, to 

 make an exception to this to a certain extent at least; these I shall class as 

 a special sub-family under the name of lymnocaudiin^. The animals of 

 these, — judging from the few species found living, — exhibit very marked differ- 

 ences from those of the marine Cardia. The siphons in Didacna, Eichw., which 

 is the principal genus of the sub-family, are elongated and united nearly up 

 to the end, and the foot is laterally compressed and broad at the base ; thus the 

 external character of the animal is that of the IIyid.e, specially recalling 

 Lidraria or 'Pholadomya. With the latter the form of the shell also agrees, except 

 in the one point, that it has no cartilage ; bvit it seems by no means without reason 

 that our shells have been by several naturalists previously classed with Fholadomija 

 in the same genus. P^omer lately (Martini and Chem. Syst. Conch. -kabinet, 

 vol. x, 1869, p. 12,) inclines to Gray's suggestion that these forms are nearly allied 

 to Funopoia and Cyrtodaria. There can be little doubt that, if we look upon the 

 shell of Didacna, with its animal, independently of all other allied forms of Cardia, 

 its systematic place can hardly be better than that indicated by Gray. However, 

 when we come from the series of marine tertiary beds of Eastern Em-ope contain- 

 ing such elongated forms of cardiinm, as Acantliocardium latum and Cerastoderma 

 edule, gradually to a series of brackish-water and then to almost purely fresh-water 

 deposits, which contain forms of cardiinm very similar to the last ones, but of 

 thinner structure, a posterior gape, sometimes with a pallial sinus, and some of them 

 possessing cardinal and lateral teeth, perfectly characteristic of the above named 

 sub-family, the question arises whether we are entitled to separate the latter as a 

 thoroughly distinct group of shells, or not. Any one who has seen these deposits (the 

 Sarmatic group) and studied its fauna will, I firmly believe, say that such a thing 

 is impossible. There are some peculiarities which these fresh-water Cardia have of 



3d 



