212 _ CEETACEOUS PELECYPODA 



The species referred to this genus are all from the upper tertiary, chiefly fresh- 

 water deposits of Eastern Europe and of Western Asia. I have already pointed out 

 the general characters which seem to justify the conclusion that the animals were 

 of a form similar to those of the next genus. 



13. Didacna/Eichw., 1838, (Bull. Soc. imp. Mosc.,p. 166). Shell elongated, 

 laterally compressed, inequivalve, usually of thin structure, surface radiately 

 ribbed ; hinge with one or two cardinal elongated teeth in each valve, sometimes 

 becoming quite obsolete, lateral teeth none ; pallial line often sinuated posteriorly ; 

 type, Cardkim trigonoides, Pallas, a recent species from the Caspian sea. 



I adopt here the name Bidacna^ as the general generic name, not only because 

 it was the first one proposed by Eichwald for this peculiar group of shells, but 

 because it seems to me to be the most appropriate one. In Didacna proper 

 the cardinal teeth are scarcely different from those of Lcevicm^dium, for in this 

 as in several other cardiinm the upper cardinal tooth of the right valve often 

 becomes nearly obsolete. In Eichwald' s Monodacna (1. cit., p. 267,) one tooth is 

 said to occur in each valve, but in reality the difference from Didacna is scarcely 

 perceptible. As usually, the right valve has one tooth particularly strongly deve- 

 loped, and in the left valve the posterior one is not distinctly separated from the 

 fulcrum. Again, in some forms the anterior cardinal tooth also becomes obsolete 

 or nearly so, but the pit for the laminar tooth of the right valve remains, and we 

 have then Adacna, Eichwald, fex parte), of which the author (1. cit., p, 169,) says, 

 '' cardo edentulus aut callus dentis loco, foveola adjecta laminaque post callum 

 elongata, ^c!' In some other forms of Adacna the cardinal teeth have become 

 perfectly obsolete ; however, even in such case young shells appear to have them indi- 

 cated. The transition of these forms, called Didacna, Monodacna, and Adacna, is 

 so gradual that it appears really very difficult to fix a limit between one and the 

 other, but if we retain the two last named ones as sub-genera, we must have a new 

 name for those forms which have a laminar cardinal tooth in the right and a simple 

 pit in the left valve. Good series of living species are necessary to settle this 

 question. 



List of cretaceous species of Carditdje. 



With regard to the cretaceous species of this sub-family I could only repeat what I have already 

 stated of several others. Thick shells, especially cast specimens, are objectionable to base specific 

 characters upon. 



See Pictet and Campiche, Pal.- Suisse, 4^^ ger., 3"^^ partie, 1866, p. 265, &c. 



1.— C Auhersonense is an Acanthocardium. 



2-3. — C. Cottaldinum and VoUzii belong apparently to the sub-genus Peduncidus, 



^ H. and A. Adams place Card. Australiense and donaciforme, wMch are said by Romer to be only varieties of 

 one and the same species, under Didacna. This is, as I already mentioned, a mistake. Card, donaciforme has lateral 

 teeth, is a solid apparently purely marine sheU, entirely of the Donax type, and does not, I think, belong at all to the 

 CARDliDm. The entire pallial line seems to be almost the only distinction from Serrtda and other allied sub-genera 

 of Bonax. The general character of the shell of the so-called C. donaciforme greatly recalls the fossil Tancredia, Lye, 

 (or Hettangia, Terq.). which I have described in the DONACIB.^, and it does not appear improbable that this fossil 

 genus is the prototype of the recent forms. Generically they appear to be hardly different. 



