100 



PJIOCEEDINGS OF THE GEOLOGICAL SOCIETY 



[Jan. 9, 



hood, and the result of the melting of masses of ice formed in the 

 winter in the higher grounds, and sliding down into the lower at 

 the break-up of the frosts in the spring. This hypothesis would 

 explain its unconformity to the beds on which it rests. The iden- 

 tity of physical conditions, then, as well as the evidence afforded by 

 the Tables of the fauna given below, proves that Mr. Searles Wood's 

 view, that the Grays Thurrock deposit is of later date than the rest, 

 is altogether untenable, and that Professor Morris was right when, 

 in 1838, he united them into one group. 



5. Testacea. — Let us now pass to the evidence afforded by 

 the Testacea. The following list of shells, partly fluviatUe and in 

 part terrestrial, is made principally from my own notes taken in 



List of Testacea from the Lower B rich-earths. 



Freshwater Species. 



Corbicula fluminahs 



Cyclas cornea, Linn , 



Pisidium amnicum, Mull , 



Unio pictorum, Linn 



U. litoralis, Drap , 



Anodon cygneus, Linn 



Limnsea peregra, Miill. 



L. auricularia, Linn , 



L. stagnalis, Linn 



Planorbis carinatus, Mull 



P. covneus, Linn 



Hydrobia marginata, Michaud. . 



Paludina vivipara,. Linn 



Bythinia tentaculata, Linn 



Ancylus fluviatilis, MUll. 



Yalvata piscinalis, Milll 



Terrestrial Species. 



Helix hispida, Linn 



H. nemoralis, Linn 



H. caperata, Mont 



Pupa marginata. Drap 



Caryehium minimum, Mull. 





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each of the pits, partly from the valuable collections of Dr. Spur- 

 rell, Mr. Grantham, Mr. Brady, and Dr. Cotton. The fluviatile 

 shells were found in groups occupying, for the most part, in death 

 the spots on which they had lived on the then bottom of the river. 

 The land-shells were comparatively rare and scattered about, and 

 probably were swept down by the floods in the same manner as the 

 mammalian bones. All the species are now alive in Britain except 

 Unio litoralis and Corbicula Jluminalis, the former being now found 

 in the rivers of Auvergne, and the latter ranging from the river Nile 

 throughout the streams of the central plateau of Asia and the Himalayas 



