THE BROWN OR COMMON RAT 615 



relatively larger and more powerful than in rattus. The zygomatic 

 arches are therefore heavier, and the masseteric plate is wider in 

 proportion to its height. The temporal fossae 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 E. r. rattus 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 

 bullae are rather smaller, and the anterior part of the basi-occipital 

 is relatively a little wider. The processes of the premaxillae 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 E. rattus. 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 X, and is fused with the latter from a relatively early stage of 

 wear ; cusps 4 and 5 are also smaller than in rattus, and more intimately 

 connected with y and s respectively. Cusps i, 4, and 5 can all be 

 distinguished in ni^ and m^ when slightly worn, and 4 is rather well 

 developed, though smaller than in rattus, in m^. In the lower teeth also 

 the outer row is more reduced than in rattus ; in m-^ it is represented 

 by a minute cusp 6 ; in m^ and m^ cusp n is present though small, 

 and in slightly worn examples of the m^ the posterior lobe is seen to 

 consist of two intimately connected tubercles — y and 4. 



Some details of the osteological diiiFerences between norvegicus 

 and rattus are given by de I'lsle {pp. cit, 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 mammae, as is well known, are 

 rather variable in number in norvegicus, and also in rattus. Hossack 



found the typical ^ ^ formula in eleven out of nineteen specimens of 



3—3 

 norvegicus examined; the formulae found in the remainder were: in 



' Fig. 91 though giving a good general idea of the skull, does not portray the 

 normal appearance of the occipital region in this species. 



