, 
THE BROWN OR COMMON RAT 615 
relatively larger and more powerful than in vatiws. The zygomatic 
arches are therefore heavier, and the masseteric plate is wider in 
proportion to its height. The temporal fosse are more extensive, 
the parietal crests, which continue the supra-orbital ridges backwards 
to the hinder edges of the squamosals, running at a higher level 
(in an old skull of Z. ~. vattus the greatest distance between these crests 
equals 88 per cent. of the cranial width, but in a similar skull of 
norvegicus it equals 73 per cent. only); the greatest distance between 
these crests is about equal to the length of a parietal measured along 
a crest. The posterior border of the interparietal is nearly straight 
instead of boldly convex, and the backward deflection of the central 
part of the lambdoidal crest is correspondingly slight." The auditory 
bulla are rather smaller, and the anterior part of the basi-occipital 
is relatively a little wider. The processes of the premaxillez supporting 
the nasals in front are relatively small. 
The mandible is of normal murine form, with large angular and 
coronoid processes; the incisor roots produce well-marked though 
small humps on the outer sides, below the coronoid processes; the 
latter rise considerably above the condyles. 
The cheek-teeth (Plate XXVIII, Fig. 9) are slightly more 
specialised than those of #. vattus, In the upper molars the median 
tubercles are somewhat increased in size, while the outer row is more 
reduced and the tendency towards lophodonty is more marked. In 
m there is usually an anterior basal cingulum ; cusp I is much smaller 
than +, and is fused with the latter from a relatively early stage of 
wear ; cusps 4and 5 are also smaller than in vat¢/us, and more intimately 
connected with y and z respectively. Cusps 1, 4, and 5 can all be 
distinguished in mm? and 7* when slightly worn, and 4 is rather well 
developed, though smaller than in vattus, in m*. In the lower teeth also 
the outer row is more reduced than in vatéus ; in m2, it is represented 
by a minute cusp 6; in 7, and mm, cusp ” is present though small, 
and in slightly worn examples of the 7, the posterior lobe is seen to 
consist of two intimately connected tubercles—y and 4. 
Some details of the osteological differences between morvegicus 
and rattus are given by de I’Isle (of. c#z., 219); most of the bones of 
the former differ from those of the latter species in showing larger 
surfaces for muscular attachment. 
Exceptional variation:—The mamme, as is well known, are 
rather variable in number in xorvegicus, and also in vattus. Hossack 
found the typical a formula in eleven out of nineteen specimens of 
norvegicus examined; the formule found in the remainder were: in 
1 Fig. g1 though giving a good general idea of the skull, does not portray the 
normal appearance of the occipital region in this species. 
