1854.] PRESTWICHj LONDON CLAY AND BRACKLESHAM SANDS. 443 



Number of Species ranging 



species. upwards. 



Mammalia G 



Aves 4 



Reptilia 23 2 



Pisces 84 10 



Mollusca 280 68 



Articulata. , 33 5 



Echinodermata 17 2? 



Zoophyta 10 1? 



. Foraminifera 28 ? 



485 "88 



§ 2. On the Amount of Relation of the London Tertiary Strata to 

 the Cretaceous Series. 



In viewing the London Tertiaries as a group, and comparing them, 

 as usual, directly with the underlying Chalk, it is to be observed that 

 we are not comparing like terms of the two periods. That a great 

 and essential difference existed between these periods must be ad- 

 mitted, but it is a question how far that difference is widened by the 

 comparison being instituted between the deep and open sea deposits 

 of the Chalk, and the littoral, shallow-sea, andflu^iatile deposits of the 

 Tertiary period, instead of between strata deposited under like condi- 

 tions during those two periods. As an indication of how these dif- 

 ferences tend in many respects to lessen in force when we have an 

 approach to more equal conditions, may be instanced the Thanet 

 Sands and their equivalent in Belgium' — the Lower Landenian system 

 of M. Dumont. Li these beds no mammalian remains have been 

 found, and, with the exception of Calyptrcea, Glycimeris, and Saxi- 

 cava, the other genera, amounting to twenty-one in number, are such 

 as lived in seas of little depth during and before the Cretaceous 

 period ; the species it is true are different, but they still — those of 

 the Lower Landenian especially — present Si fades so closely resembling 

 that of analogous groups of certain lower cretaceous strata, that 

 some eminent palaeontologists and geologists are inclined to consider 

 the Belgian beds as more Cretaceous than Tertiary. Where more 

 shallow-water conditions again prevailed during the later Cretaceous 

 periods, as at the time of the deposition of the Calcaire pisolitique, 

 the fauna which then flourished presents forms so closely resembling 

 those of the overlying tertiary strata, that the opinions of some of the 

 most able geologists of France have been, and still are, divided, as to 

 which of the two periods this deposit should be referred*. I mention 

 these facts, not as attempting to ignore the value of the distinction 

 drawn between the Cretaceous and Tertiary periods, but for the purpose 

 of suggesting whether that distinction is so extreme as we have con- 



* The shells were at first all referred to tertiary species. A closer examination 

 lias however detected specific diflferences, although it is apparent that, with respect 

 to the genera, the analogy must be strong to have given rise at all to such an 

 opinion. The question is still under discussion. 



