206 



CRETACEOUS PELECYPODA 



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

 Lymnocardium. Detailed characteristics will be given farther on. The reasons 

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

 recent DidaoKB 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, Prance, 1™"' ser., vol. iii, 18G8, p. 1, &c.), we find that there are 

 a nvimber 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 Bi- or Ilono- or 



A- dacna. 



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

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

 examination of more species and their comparison with marine Curdia 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 ztmnocardiin^ are by no means rare. In the 

 SOLUJNIDM, for instance, we have Pharella with united siphons and NovacuUna with 

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

 in both almost identical. In the IIyidje we have 3Iya and Anatina with long 

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

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

 and the mactrin^ on the other hand ; and again the venerinm and the dosiniin^, 

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

 And ao'ain the fresh-water genera Sphcsrium and Pisidlum have the siphons more or 

 less vmited and elongated, while the brackish Cyrence 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 Bidacna 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 Bidacna have as yet been examined, and, on the other hand, those of by no means 

 all the species of cardiin^, particularly of the elongated Pajjyridece, (with pallia! 

 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 wiU also be found to 

 be the case with the animals. 



* Some of which appear to be of more reoent date than the true "Congerien Schichten" of the Vienua aud Hun- 

 garian basin. 



