2321] Freshwater from Marine Shells. 311 



belono- to river shells when deprived of their epidermis ; these 

 specimens consequently must have been covered by it, not that 

 iCeaTto advance the presence or absence of an epidermis as a 

 distinctive character between freshwater and marine shells ; but 

 I have iood reason to believe, that this peculiar kind of epider- 

 misTnot found on the marine Cerithia,* having never seen any 

 Z such as have hitherto come under my observation This 

 epidermis appears to be quite necessary to the Potaimdse and 

 indeed to freshwater shells in general, to enable them to resist the 

 action of the water ; for we find that wherever, by any accident, 

 or circumstance, this epidermis is worn off or otherwise gone, 

 the shells are eroded, sometimes very deeply ; and this happens 

 to t iose parts of the shells which have been for the longest 

 space of time exposed to the action of the water, such as the 

 Zboi.es of bivalves, and the point of the spire "*^**£ 

 and, which may appear singular, the water of the sea does not 

 seem to possess the same power of corroding those parts of shells 

 which.having lost their epidermis, are thus exposed to its action— 

 if 1 may judge from a specimen of a bivalve shell now before me 

 which has lost its epidermis at its umbones and its most promi- 

 nent parts, and remains otherwise unaltered It is also to be 

 remarked, that this erosion takes place upon the more prominent 

 parts, and the parts thathave been longest exposed to the action 

 of the water, not only in rivers, but in lakes and that it is com- 

 mon not only in the immense continental lakes, but in some : of 

 those of our own island : in fine, it is so common in shells that 

 are certainly of a freshwater nature, that I consider it as one ot 

 the best characters by which they may be distinguished from 

 marine; and particularly so as I do not recollect any instance of 

 such erosion in any shells that are decidedly marine. There are 

 however, two accidents to which marine shells are subject; the 

 one is, that of being grown over by an irregular crust of corallmd 

 matter; the other, that of being pierced by immense multitudes 

 of minute vermes; both these accidents produce an appearance 

 somewhat resembling the erosion above spoken of, but may be 



* Although some of those univalves which in the general contour of their ^shells 

 resemble the species of the decidedly marine Genus Cerithium, are undoubtedly the 

 ntbhi^tCmouths of rivers ana situations where the fresh - S J-^ 

 become raised, it is not, therefore, to be supposed, that all are ; for there are 

 SnL Similar shells 'from the interior of Africa, and in some « jSWgfflg 

 rivers far above the point of union between the salt and freshwater Their Presence, 

 Therefore, tn a stratum, maybe considered rather as an evidence of its freshwater than of 

 tsnSe origin. In the Danube, at Waitzen, in Hungary, a «'— ™ e J^™» 

 appear still more extraordinary, there is a bioalve so exactly similar m its Generic Cha- 

 pters to some of the common^ed Mytili (of Lamarck) that from ^W *» 

 impossible for me to mention any one in which it differs. I say I do not ^^^ 

 Generic Character in which it differs from decidedly marine specie ; but 1 am never 



X" persuaded, that if we knew its animal inhabitant, or had opportunity of Uidying 

 Un Native situation, we should soon find some ^tti^S^S 

 a* well as habits, of the animal, and probably also m the form ot JSjgJvJJJfli 

 the least doubt that this will prove to be the case as well in the Ccnth .o.d shcUs as «. Uie 

 bivalves, as we know it to be already in such as we have the meat* ot observing. 



