1854.] PRESTWICH, LONDON CLAY AND BRACKLESHAM SANDS. 447 



London Clay, with the exception possibly of the Teredina, which 

 however is probably only a subgenus of Teredo. 



Unlike the succeeding Tertiary period, the number of Mammalia 

 in the London Tertiaries is very limited. One genus (Macacus) is a 

 form now existing, whilst the Hyracotherium and Coryphodon are 

 extinct and peculiar to the London Clay. The LopModon is a genus 

 which, scarce in this period, becomes numerous in the overlying 

 group. No mammalian remains have hitherto been found in the 

 lowest division of the London Tertiary group,^ — the Thanet Sands. 



The remains of birds are too few and too fragmentary to have much 

 specific value ; nor, as bones of birds are found in the Wealden, are 

 they of importance with respect to their presence as a class. 



The large proportion of Reptiles is a peculiar feature ; they 

 mostly belong to families and genera which have a prolonged vertical 

 range from the Wealden strata upwards. Although the peculiar and 

 characteristic Cretaceous genera are absent, the prevalence neverthe- 

 less of Reptilian forms and the scarcity of Mammals are not without 

 a certain significance. 



§ 'S. General Considerations. 



It will be seen from the foregoing examination, that the relationship 

 of the London Tertiaries with the Bracklesham Sands is maintained 

 by several specific identities, and is therefore one comparatively of no 

 distant degree ; still the generic differences of the fauna which are 

 so marked in the tertiary strata of the age of the Bracklesham and 

 associated series, as compared with the cretaceous strata, are cer- 

 tainly less prominent in the strata of the London Tertiary group. 

 Although it is true that with one or two exceptions there are no 

 species in common to the two latter series, yet the fades of the fauna 

 often presents points of strong resemblance, and a not inconsider- 

 able number of genera are common to the two periods*. It is to be 

 observed also, that in the London Tertiaries not a single recent 

 species is found with the exception of the Terehratulina striatula, 

 which species however is common also to the Cretaceous period. 



Taking all these facts into consideration, I hold the distinction 

 between the London Clay and the Bracklesham Sands not only to 

 be sufficiently evident upon physical grounds, but to be equally well 

 based on the independent character, in the main, of their organic 

 remains. All the phsenomena point to an original difference in the 

 direction of the seas and position of the land. The Loudon Tertiary 

 group seems to have resulted in that order of changes, which, com- 

 mencing vdth the elevation of a portion of the Chalk area at the end 



* We know how difficult the separation of the cretaceous and tertiai'y deposits 

 is in some parts of Soutliern Europe, where the strata of these two periods present 

 similar Uthological characters, and exhibit like conditions of hfe. Might we not 

 therefore expect to tind similar or even greater resemblances in these possibly 

 still lower strata of the London Tertiaries, were the mineral character assimilative 

 instead of divergent, and were the terms of comparison alike, instead of being 

 in the strong contrast in which they occur in this more northern part of Europe ? 



