206 CEETACEOUS PELECYPODA 



their own, as noted above, and these induce me to propose for them the name 

 Lymnocardium, Detailed characteristics will be given further on. The reasons 

 which make it most probable that the Lymnocardia have to be associated with the 

 recent DidacncE of Eichwald must for the present be taken from geological 

 evidence. In looking over the fauna of the Sarmatic deposits* of the Crimea (see 

 Mem. Soc. Geol., Erance, 1^' ser., vol. iii, 1868, p. 1, &c.), we find that there are 

 a number of Crimean Lymnocardia specifically the same as those occurring in 

 similar beds in Southern Hungary. With these there are, however, other species 

 associated which do not possess lateral teeth, but either two or one, or no cardinal 

 teeth, and thus are in every way identical with Eichwald' s Di- or Mono- or 

 A' dacna. 



These data, I think, sufficiently justify the classification of the lymnocardiin^ 

 with the other carbiinjb in one family ; and it yet remains to be shown by the 

 examination of more species and their comparison with marine Cardia whether the 

 distinction of the animals of Bidacna are really so important and constant as to 

 overthrow all the above arguments regarding the classification of these shells. We 

 know from the account given of other families that changes similar to those pointed 

 out in the animals of cardiin^ and lymnocabbiin^ are by no means rare. In the 

 soLF.NiBM, for instance, we have JPharella with united siphons and Novaculina with 

 perfectly separated siphons, while all the other characters of animal and shell are 

 in both almost identical. In the Myibm we have My a and Anatina with lono- 

 united fForomya, ThraciaJ, and others with more or less shortened and partially 

 divided, siphons. Speaking of larger groups we can refer to the lutrariinm on one 

 and the mactrinm on the other hand ; and again the venerin^ and the bosiniin^, 

 the latter having long united siphons, perfectly like those of the lymnocarbiin^. 

 And again the fresh- water genera SplKjerium and Fisidium have the siphons more or 

 less united and elongated, while the brackish GyrencB of the same family have 

 them very short and separated. 



These (and many other instances which could be cited) distinctly show that the 

 observation of Didacna having a united siphon, is by no means an isolated example, 

 but that similar changes of development are most common among many other 

 families of Pelecypoda. When we consider that the animals only of a few species 

 of Didacna have as yet been examined, and, on the other hand, those of by no means 

 all the species of cardunm, particularly of the elongated Fapyridecs, (with pallial 

 sinus), we can hardly be justified in stating that no intermediate forms between the 

 last named sub-family and the lymnocardiin^ exist. The shells unquestionably 

 indicate them, and there is every reason to believe that such will also be found to 

 be the case with the animals. 



* Some of which appear to he of more recent date than the true " Congerien Schichten" of the Vienna and Him- 

 garian basin. 



