THE FOULA FIELD MOUSE 731 
A. f. thuleo, while quite closely related to the other members of the 
sylvaticus group, is a very clearly defined insular form, best treated as a 
sub-species of frzdariezszs. 
Since the “key” to British MWusvide, at p. 377 above 
was drawn up, our knowledge of the British members of 
the genus Afodemus has greatly advanced. It is now known 
that of the five species inhabiting these islands, three, A. sy/va- 
ticus, fridarvensis, and hebridenses, have been differentiated 
into a number of more or less well-marked insular races or 
sub-species. A. flavicollis, confined to southern Britain, 
and A. hertensis, inhabiting St Kilda, are each represented 
by a single form alone. The precise determination of Long- 
tailed Field Mice from the small islands off the British coast 
is a matter of considerable difficulty, calling for great patience, 
skill, and an accuracy of skull measurement and calculation, 
which in most cases will probably be beyond the powers 
of an ordinary field naturalist. It is impossible to frame a 
satisfactory ‘‘key” based upon either the external or the 
cranial characters alone; but the following ‘“key,” based 
upon both sets of characters together, sums up our present 
knowledge of this most difficult group and will probably assist 
future investigation :— 
“KEY” TO BRITISH SPECIES AND SUB-SPECIES 
oF APODEMUS. 
(A) Skulls of adults with post-molar region relatively long; the distance 
between a condyle and 3 equalling 45.5 to 48.3 per cent. of the 
condylo-basal length, the average value of this dimension never less, 
and usually more, than 46.23 per cent. of the condylo-basal length. 
(2) Skull with feeble temporal ridges, and smoothly rounded brain-case ; 
incisive foramina relatively long, their length from 21.9 to 25.5 per 
cent. (average 23 to 24 per cent.) of the condylo-basal length. 
Pectoral spot not developed as a collar. 
(a) Size smaller (head and body about 95, 
hind-foot about 22 mm.). 
(a) General dorsal colour brighter, tail 
longer, averaging more than 95 per 
cent. of the length of headand body. sylwaticus sylvaticus. 
VOL. Il. 3A2 
