1854.] PRESTWICH, LONDON CLAY AND BRACKLESHAM SANDS. 439 



his valuable work on British Fossil Mammals, mentions only two 

 mammals from the London Clay, and one from the Bracklesham 

 sands*; they are distinct species. 



The remains of four species of Birds are found in the London 

 Clay, but the Bracklesham series offers in this class no terms for 

 comparison. 



The Reptilesf constitute an important and well-characterized group 

 of 2 1 species, of which number two only have been identified among 

 the 7 Bracklesham species ; and one of these, the Chelone longiceps, 

 must be received with a doubt, owing to the imperfect character of 

 the specimens upon which the determination rests. In the families 

 occurring in the two deposits there is a marked concordance. 



The Fishes of the London Clay are equally characteristic and still 

 more abundant, amounting to as many as 83 species ; whilst in the 

 Bracklesham sands 28 species have been recognised ;];. Of these, 

 although several of the genera, as Myliobatis, Mtobatis, Otodus, 

 Lamna, are well represented in both, only 10 species are common to 

 the two deposits. It will be observed, that a great point of re- 

 semblance in the two groups is the abundance of cartilaginous fishes 

 of the Shark and Ray tribe, and that the distinctive feature is the 

 presence in the London Clay of a considerable number of Ganoidei, 

 an order which is entirely wanting in the Bracklesham series. 



Of the MoUusca we find a larger proportion common to the two 

 periods. In 1847 the known species altogether numbered only 283, 

 and of these 43, or about 15 per cent., were considered common to the 

 two deposits. Mr. Edwards has had the kindness to furnish me with a 

 list, which is appended to this paper, of the Bracklesham (see p. 450) 

 and London Clay species in his collection, and has also greatly assisted 

 me with his opinion as to the species common to the two series §. The 

 total number of determined species now amounts to 540, of which 

 316 are peculiar to the Bracklesham beds, and 168 to the London 

 Clay, while the number common to both does not exceed 56 species, 

 or under 10 per cent., a proportion which may possibly be further 

 reduced when a larger number of species are accurately determined. 

 Of the Cephalopoda there are 1 species in the London Clay, and but 

 8 in the Bracklesham beds : of these 4 are common to the two series. 

 Of Gasteropoda there are, on the contrary, but 138 species in the 

 former deposit, and 232 in the latter : of these 37 species are common. 

 Nearly the same proportion holds good in the Lamellibranchiata, of 

 which there are 70 species in the one, and 127 in the other ; of these 

 14 are found in both. Of Brachiopoda there are 2 species, of which 

 one is in common. There are 4 species of Bryozoa in the Brackle- 

 sham beds, and 4 in the London Clay ; they are all distinct, but one 



* It is not until we get higher in the Middle Eocenes that mammals become 

 numerous. 



t Mon. Palaeont. Soc, vol. for 1850-51 ; Owen and Bell, " Fossil Reptiles of the 

 London Clay." 



t Agassiz's " Poissons Fossiles," and Trans. Brit. Assoc, for 1844, p. 307. 



§ I am also indebted to Prof. Forbes and Mr. Morris for their aid in the critical 

 discussion of this subject. 



VOL. X. PART I. 2 H 



