1821.] of Headen Hill, Isle of Wight. 219 



Madagascar, the River Congo, &c. There is also found in 

 «reat profusion a species of freshwater shell very like one 

 known by the name of melanopsis buccinoides, and which, I 

 think Mr. W. may have mistaken for ancilla buccinoides. The 

 only shell I can find in this stratum which at all resembles murea 

 reticulars is a small but beautiful species of melania. I have 

 mentioned above my doubts about natica caurena being found 

 in this stratum. I think a true freshwater nerita nearly related 

 to Viro-inea may have been mistaken for it. It is remarkable 

 that this nerita retains its colours almost as strongly as the recent 

 shells of North Virginea; it is very abundant, and with the 

 melanopsis abovementioned, and some others, it is found in the 

 blackest coal-like layers of the formation, the shells in which 

 are almost always in a very tender state, and fall to pieces on the 

 slightest touch. 1 do not know the shell which Mr. W. calls 

 cytherea Scutellaria ; neither am I acquainted with his " bivalve, 

 apparently of the genus erycina," or his " murex nodulanus. 

 The cytherea and the erycina may possibly be cyclades, or 

 cyreme, and the murex may prove to be a melania. Of these, I 

 cannot say any thing certainly, not having met with them. The 

 few marine shells I found were principally oysters ; a species ot 

 buccinum, voluta spinosa, and ancilla subulata ; the last three 

 very sparingly indeed. Of these I should have found it more 

 difficult to dispose, had not Mr. W. informed methat " occa- 

 sional mixture of marine and freshwater shells may be expected,' 

 and that " they would denote either the gradual nature of the 

 change that has taken place in an arm of the sea before it 

 became completely a lake of freshwater, or the occasional irrup- 

 tions of the ocean at a subsequent period." I must, however, 

 observe, that the oysters are only found here and there in small 

 patches, and that they are not only mixed with those of whose 

 freshwater nature it is difficult to speak decidedly, but with li/m- 

 nm and paludina:, which Mr. W. has not noticed in this stratum, 

 though, I believe, he has noticed the same species of paludina, 

 under the term cyclostoma, in the " freshwater formations. 

 This is very abundant in some parts of this stratum, and is 

 accompanied, also, by the shell which I suppose to be potamides 

 Lamarckii,* one of the commonest in the stratum. 



1 think I have now nearly proved this stratum to be rather ot 

 freshwater than of marine origin. Thus much is certain, that by 

 far the greater number of shells found in this stratum are fresh- 

 water, and not marine ; so that " if we depend upon fossils as a 

 principal means of identifying strata," we shall see great reason 

 to believe that there does not exist any marine formation between 



• I may be mistaken about tbe name of this shell, because I have no means of ascer. 

 taining it with certainty, Mr. W. not having referred to any description of it. 1 con- 

 jecture it to be so from its resemblance to a cerithium, and from its possessing tne 

 characters of some shells which I know to be inhabitants of freshwater, and also resem- 

 bling ccrilhia. 



