310 Mr. Sowerby on the Means of disth fguishing [Oct. 



in this series of beds. Had they ever existed, I think their 

 remains would have been evident, considering how much thicker 

 and stronger marine shells in general are than those of fresh- 

 water.") This is quite a mistake. Marine shells are not gene- 

 rally thicker in their substance than freshwater shells ; it is true 

 that many of the marine shells are very thick, and comparatively 

 much thicker than a great many freshwater shells ; but then, on 

 the contrary, a great number of the freshwater shells are also 

 very thick, and comparatively much thicker than many marine 

 shells, and some of the marine shells are decidedly among the 

 most delicate and thin ; and we shall find this at best but a 

 variable character, greatly dependent upon accidental circum- 

 stances, such as age, disease, situation among rocks, &c. and 

 the same species is sometimes very thin, at others, very thick. 

 Many shells which are thin in their young state increase in thick- 

 ness by age ; disease is also the cause of extraordinary thickness 

 in some cases ; and Testaceous Mollusca which live in the rocky 

 beds of rapid currents are obliged to accommodate themselves 

 to circumstances, and thicken their shell accordingly. 



The epidermis, or, at least, the sort of epidermis which gene- 

 rally coats freshwater shells, appears to be a character deserving 

 of some consideration ; but though I believe all freshwater shells 

 have an epidermis, yet I do not think it is always the same sort 

 of epidermis : that is, it appears to vary in thickness as the shell 

 it coats varies ; the thin and more delicate shells, such as the 

 Limnei, having a thin, nearly transparent epidermis ; while the 

 thicker shells, such as the Uniones, have a thick and almost 

 black epidermis. Geologists, however, seldom have it m their 

 power to observe the epidermis in fossils, though this is some- 

 times the case : an instance is upon record which I must here 

 mention. Brongniart says : " On trouve encore ce Potamide* 

 dans le foret de Montmorency au dessus de St. Leu ; il y a con- 

 serve son test et la couleur brun roussatre qui appartient gene- 

 ralement aux coquilles fiuviatiles." Now this colour does not 



•* I cannot avoid noticing the fact, that Mr. Webster has charged Lamarck with hav- 

 ing instituted a Genus, founding its distinctions not on any difference in its form, but on 

 the difference of the water in which it had lived. Mr. W.'s own words are to be seen 

 in Geol. Trans, vol. ii. p. 230, second note. The truth, however, is, that the Genus is 

 founded by Brongniart, and not by Lamarck. Brongniart's words are: " Nouspropo- 

 sons d'etablie ce Genre qui est fonde plutot sur les habitudes des animaux qu'il renferme, 

 que sxtr Uimportancc des caracteres exterieurs ; il differe, en effet, tres peu des Cerites; 

 mais on remarquera que dans le Genre des Cerites, etabli par Bruguiere, il y en a qui 

 sont habitantes ou des marais voisins de la mer, ou des eaux saumatres de l'enlbouchure 

 des fleuves ; or celles de ces especes que nous avons vues ont les caracteres que nous 

 attribuons au Genre Potamide, aussi avoient elles ete toutes place dans la division du 

 Genre Cerite qui a pour caractere distinctif un canal droit et tres court." 



I might very reasonably insist upon the superior importance of characters drawn from 

 the form and habits of the animal inhabitants of shells ; yet it will appear that this Genus 

 is not founded merely upon the supposed fact of its having inhabited, freshwater, since 

 we see that it possesses some particular external marks of distinction, marks which cha- 

 racterize all those shells found in situations which may from analogy be supposed to 

 resemble that in which this fossil occurs. 



