Vol. 50.] FAUNA FROM THE IGHTHAM FISSURE. 205 



been found in this fissure : Rhinoceros antiquitatis, Elephas primi- 

 genius ?, and the new species of Mustela ana Mus • but it must be 

 remembered that the number of extinct mammals comprised in the 

 Pleistocene fauna is comparatively small. Of the 48 species noticed 

 by Prof. Boyd Dawkins in 1869, only 11 are extinct. 



No doubt much uncertainty exists as to the exact time when 

 some of the smaller mammals became extinct in this country, for 

 unlike the larger animals (such as the urus) which were noticed by 

 very early writers, such small forms as the northern vole and 

 Arctic lemming would not attract attention, even though they 

 should have continued in the country to much later times. It is 

 unlikely, however, that species now restricted to steppe and Arctic 

 conditions would have continued to live in Britain when the 

 climate became temperate. Moreover, there is no evidence of their 

 remains having been found in alluvial or other recent deposits. On 

 the other hand, it is quite certain that the smaller mammals were 

 more capable of adaptation to extreme changes of condition than 

 were the larger ones. The field-mouse and some of the voles 

 which lived in Britain in the times of the Norfolk Forest Bed are 

 living here at the present day. Pew, if any, of the larger species 

 have been able to maintain their position ; but this no doubt is 

 largely due to the influence of man. 



A mere glance at the list of species from the Ightham fissure shows 

 how large a proportion of them are forms now living in the district 

 (about 32), and these give so recent a look to the entire series that 

 it may naturally be questioned whether this deposit is anything more 

 than a recent accumulation. The fact that some extinct forms are 

 found mixed with the recent species is not in itself a sufficient answer 

 to this question. At first sight it seems quite possible for the extinct 

 forms to have been derived from some Pleistocene deposit on the 

 higher ground, which has now been entirely removed by denuda- 

 tion, and these, being gradually washed into this fissure from above, 

 might have been re-deposited with the recent species in quite modern 

 times. There are, however, two reasons which militate against 

 this supposition ; in the first place, if this were the correct inter- 

 pretation, we should expect to find the two series of bones in a very 

 different state of preservation, the derived bones being rolled and 

 denuded ; but such is not the case, for although one or two speci- 

 mens have been partially dissolved, apparently by exposure to 

 percolating water, the Rhinoceros teeth are as sharp and perfect as 

 they can well be ; the large Rhinoceros humerus snows no signs of 

 having been worn, to any appreciable extent, since it was gnawed ; 

 the elephant foot-bone is likewise unworn. And, further, the skulls 

 and bones of the small species, such as the lemmings and northern 

 voles, which are not known to have lived in this country since 

 Pleistocene times, are just as perfect as those of the recent species 

 found with them. The second argument against the deposit being 

 recent is derived from the physical conditions of the locality. This 

 Assure is situated at the end of a small promontory on three sides 



