202 CASTORID&. 
below the present surface, in places where they may have 
been overwhelmed by debris, or even buried by man; and, 
although these bones are the most recent of all, they are 
almost always, owing to their superficial situation, the 
worst preserved.” * 
The fossil remains, however, of the Beaver discovered in 
the lacustrine clay with the submerged forest at Bacton, 
and those obtained by Mr. Warburton at Southwold, and 
by Mr. Lyell at Thorpe, from the fluvio-marine crag, carry 
back the date of this existing species to the pliocene ter- 
tiary period, when it was the associate of the Mammoth, 
Rhinoceros, and Hippopotamus. 
The like antiquity of another and smaller Rodent of the 
Beaver family, still existing in most of our British rivers 
and smaller streams and ditches, is more abundantly testi- 
fied by the numerous fossils of a species of Arvicola, which 
I have been unable satisfactorily to distinguish from the 
Arvicola amphibia, or common Water-rat. 
Dr. Buckland appears to have been the first to have 
noticed the fossil Avvicole in British localities, observing, 
with regard to the Kirkdale cavern, that ‘the teeth which 
occur, perhaps in greatest abundance, are those of the 
Water-rat ; for in almost every specimen I have collected 
or seen of the osseous breccia, there are teeth or broken 
fragments of the bones of this little animal mixed with, and 
adhering to, the fragments of all the larger bones. These 
rats may be supposed to have abounded on the edge of the 
lake, which I have shown probably existed at that time 
in this neighbourhood.” 
The abundance of these small aquatic Mammals in the 
Kirkdale cave, at first view suggests a very common con- 
* Cambridge Philosophical Transactions, vol. i. p. 177. 
+ Reliquize Diluviane, p. 18. 
