Sus MEI. ue ee 
of two British Species of Plecotus. 57 
manus. On the other hand, in the P. auritus the thumb is 
somewhat shorter, and the tail not so much exserted from 
the interfemoral membrane; of which last part it may be also 
added, that in the P. brevimanus its extreme tip terminates in a 
fine point, whilst in the P. auritus it is somewhat obtuse and 
flattened. Another, and perhaps the most obvious distinction, 
resides in the expansion of the flying membrane, which, viewed 
relatively as well as absolutely, is by much the more considerable 
in the P. auritus. This circumstance arises from the greater 
development of the metacarpal bones and the phalanges of the 
fingers, as compared with the arm and forearm. In the P. au- 
ritus, the length of the middle finger, or the distance measured 
from the carpus to the apex of the wing, exceeds in length the 
arm and forearm together by three lines, and the forearm taken 
separately by more than an inch; whereas in the P. brevi- 
manus the length of this part is less than that of the arm and 
forearm together, and only exceeds the forearm separately by 
six lines. It is with a view to this last peculiarity that I have 
selected the trivial name of this species. Lastly, I may remark, 
that in the P. brevimanus there is a shallow notch on each side 
of the interfemoral membrane, about half way between the heel 
and the extremity of the tail, which in the P. auritus is scarcely 
visible. 
The above distinctions, many of which are founded upon a 
comparative view of the osteology of the two species, can 
scarcely be considered as the variations of a different age. In- 
dependently of them, however, these bats, when seen together, 
. will not be easily confounded, from the great difference in their 
absolute size, and in the colour,—more especially of their under 
parts. In the P. auritus, the colour is brownish-grey mixed 
with dusky, and is nearly the same above and below, being in 
the last instance merely of a somewhat paler tint. In the P. bre- 
VOL. XVI. I vimanus, 
