232 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 25, 



Paludina lenta. Limnaea longiscata. 



Cyrena obovata. Planorbis euomphalus. 



cuneiformis. obtusus. 



Potamides concavus. Melanopsis. 



ventricosus. Melania fasciata. 



cinctus. 



and at White Cliff Bay by 100 feet of clays and marls, containing 

 also almost exclusively the Paludina lenta, Melanopsis carinata, 

 Melania fasciata and Cyrena obovata, and only a few specimens of 

 Limncea and Planorbis, 



At Headon Hill I found in one of the limestones of this series 

 (No. 69) a iew teeth and bones. The former have been identified 

 by Professor Owen as belonging to the Palceotherium crassum and 

 P, medium. 



Some of the bones were those of the Turtle, remains of which I 

 met with also in stratum No. 74 of Headon Hill. 



The distribution of the organic remains of this division will be 

 more fully noticed at the end of this paper *. 



In introducing local subdivisions and groups it may be convenient 

 not only to consider the strata, with regard to their palaeontological 

 and lithological characters, but also to attend to those traces of 

 minor disturbances effected during their accumulation, which may 

 often usefully serve as lines of separation to the different groups. 

 Inasmuch also as the effects resulting from these minor disturbances 

 afford evidence of greater or lesser changes in the hydrographical 

 condition of the district during the period when the formations were 

 in course of accumulation, making slight breaks, but not destroy- 

 ing the continuance of like genera and species, although modifying 

 their distribution, they may, I think, conveniently serve as points 

 of division in the series. This is more especially the case, since 

 these disturbances having been greater in one part than in another, 

 the changes effected necessarily differ in importance, the condition 

 of the fauna varying in the ratio of the amount of change, of which 

 the effects are likewise almost always more or less visible in the 

 lithological character of the beds. 



There has been great difference of opinion with regard to the 

 theoretical arrangement of the tertiary series of the Isle of Wight ; 

 some agreeing with Mr. W^ebster (to whom we are indebted for the 

 first systematic attempt at classification), that they can be divided 

 into " sands and plastic clays and London clay, with two freshwater 

 and one upper marine formations ;" others, on the contrary, con- 

 tending that all the members of the lower part of the series alternate, 

 rendering a division into London and Plastic clays impracticable, and 

 considering that the upper freshwater and marine strata also alter- 

 nate. 



Now I do not think that Mr. W^ebster intended to separate the 

 plastic and London clays into two formations, as we now understand 



* See also the sections and their explanations at p. 252. In the preceding 

 paragraph only the general grouping and most marked phaenomena M-e noticed. 



