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 oi fridariensis. 



Since the "key" to British MuridcB, at p. 377 above 

 was drawn up, our knowledge of the British members of 

 the genus Apodemus has greatly advanced. It is now known 

 that of the five species inhabiting these islands, three, A. sylva- 

 ticus, fridariensis, and hebridensis, 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. hirtensis, 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 W3 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. 

 (a) 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, 

 (fli) 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 head and body, sylvaticus sylvaticus. 

 VOL. II. 3^2 



