380 MURIDA 
(B) Palate complicated, with lateral pits; 
cheek-teeth rootless :— 
» 
(a) Size larger; “rats,” with condylo-basal 
length rarely less than 39 mm. ; #, with 
three closed leet : sli 
|ARVICOLA 
amphibius.* 
(B) Size smaller; “mice,” with condylo- 
basal length rarely reaching zomm.; m, -MICROTUS. 
with five closed triangles. | 
(a’) m? with four dentine-spaces :— 
(a) m, with anterior loop nearly bi- 
laterally indented :— 
(Westray Island, 
Orkneys). 
orcadensis (South 
Orkney Islands). 
(b) #, with outer infold of anterior) MZ. sandayensis sanday- 
loop much less se than ensts (Sanday Is- 
inner infold land, Orkneys). 
(a’) Cranium depressed 
[ sandayensis westre 
(b’) Cranium not depressed alee 
(B’) #2? witha small 3rd inner Eee ae 
making five dentine-spaces :— 
(a) m! with three inner salient angles ; 
bullze small :— 
(a’) Sizesmaller(condylo-basal ie 
not exceeding 26 mm.) LEAL 
(b’) Size larger (condylo-basal length| AZ. a. neglectus (Scottish 
reaching 28-29 mm.) if Highlands). 
(b) m! frequently with a small 4th inner 
salient angle; bullee large :— 
(a) Temporal ridges anteriorly\ JZ. a. macgiliivratt 
parallel in adults . 3) (Islay). 
(b’) Temporal ridges anteriorly con- ‘bas a. exsul (Hebrides). 
verging in adults : 
2. Skull rounded; cheek-teeth with tubercular 
crowns :— 
(A) Rostrum short (distance from orbital con- 
striction to anterior edge of nasals about } M@Z7CROMVS minutus. 
4 total length of skull) . . 
(B) Rostrum normal (distance from orbital 
constriction to anterior edge of nasals 
about } total length of skull) :— 
Saas 4 
(a) Upper incisors motched; m much) us. 
longer than #? and m* together 
(4) Mesopterygoid fossa ae : = 
pointed anteriorly ‘ [it muralis (St Kilda). 
(B') Mesopterygoid fossa squarely trun- ae. ee ee 
cate anteriorly 
* The sub-species of Arvicola amphibius are omitted, as they do not possess diagnostic 
cranial or dental characters. 
