546 Deposits in the Valley of the Thames 



Are then our gravel beds anterior or posterior to these flu- 

 viatile alluviums ? Or is the mammaliferous stratum overly- 

 ing the crag a marine deposition of the same period ? The 

 bones found at Whitstable being dredged from the bed of the 

 sea, it is impossible to know whether they belong to an an- 

 cient fluviatile or to a marine deposit, which extended along 

 this coast, as well as the opposite one of Norfolk and Suffolk, 

 where mammalian remains are even more abundant. The 

 connexion of these deposits with the ancient beds of gravel, 

 is a subject requiring further elucidation, and I would beg to 

 call the attention of geologists to some valuable observations 

 by Croizet and Iobert* on similar fluviatile strata in the Au- 

 vergne district, which had been referred to diluvial action, 

 and from whose work the following passage is quoted as bear- 

 ing on the question. 



" On a souvent, en effet, attribue aux eaux marines des ef- 

 fets qu'elles n'ont jamais produits. Lorsq'on a rencontre des 

 masses alluviales de plusieurs centaines de metres d'epais- 

 seur, on a cru que des eaux d'une elevation immense avaient 

 seules performer ces depots ; on n'a pas meme songe a re- 

 chercher si une partie du sol etait alors a decouvert, ou si la 

 matiere avait son origine et sa source dans le lieu meme ou 

 elle s'etait deposee, et la science ainsi detournee de sa veri- 

 table route, s'est perdue dans les systemes." 



Kensington, August, 1838. 



Comparison of Cyrena, Valvata, and Unio, found at Grays, with 

 recent Species. By G. B. Sowerby, Esq., F.L.S., &c. 



Cyrena.^ 



The nearest recent species known to me is one which 

 abounds in the canal of Alexandria. The recent one is rather 

 thinner and more perfectly equilateral than the fossil. The 

 anterior side of the fossil is comparatively shorter than the 

 same side of the recent species ; and the posterior extremity 

 is more truncated in the recent than in the fossil. All the 

 teeth are larger and more prominent, and the fulcrum to 

 which the ligament is attached, is smaller in the fossil than 

 in the recent. The curve at the anterior part of the anterior 



* ' Recherches sur les Ossem. Foss. du Dept. du Puy de Dome,' 1828, 

 p. 35, et p. 90-97. 



t Several figures of this very interesting shell are given in the * Magazine 

 of Natural History,' vol. vii. page 275. — Ed. 



