OF EXTINCT MLSCARDIXE RODENT. 213 



attenuation, its posterior portion particularly being very slight 

 and the cheek-teeth closely crowded together. This applies also 

 to the skull, though perhaps to a lesser degree. The mandible of 

 the Balearic genus is altogether ixiore robust, and gives the im- 

 pression of being possibly less highly specialised. The angle of 

 the jaw is perforated, and also resembles in shape that of Eliomys, 

 but compared with this last the coronoid process originates 

 further forward and rises more abruptly ; it tapers to a slender 

 point but is comparatively shorter and the space between it and 

 the condyle less deeply excavated : this last also applies to the 

 region between the condyle and the highest point of the angle, 

 causing the hinder portion of the jaw to be more sohd owing to 

 this comparatively greater extent of bone. The articulating 

 surface of the condyle is strongly marked. The symphysial 

 region and that between it and the cheek-teeth are likewise 

 robust. The incisor extends considerably behind and rises above 

 the cheek-teeth row, causing a marked protubex-ance on the out- 

 side of the jaw at the base of the coronoid process, and the 

 iirferior dental foramen occupies a correspondingly high and 

 backward position. 



Teeth. — The dental formula is the same as that of other of the 

 Muscardinid?e, that is to say, i. \, pm. \, m. |-. The incisors are 

 of medium size, with the anterior band of enamel smooth and 

 stained the charactei-istic orange-yellow colour. The upper 

 incisor originates above and just in front of the premolar; its 

 inner surface (PI. I. fig. 10) is flat and the outer gently rounded, 

 this being also the case in the corresponding tooth of the lower 

 jaw. Both are considerably compressed laterally with the antero- 

 posterior thickness much greater than the lateral width, this being 

 nearly double in the lower teeth. Their transverse sections (PI. I. 

 figs. 11 k 12) are more or less elliptical in shape, thus differing 

 from those for instance of Eliomys, Glis, and Leithia, in which the 

 section forms practically an isosceles triangle with the anteiior 

 face as the shorter base (PI. I. fig. 13). There is naturally a 

 corresponding difference in the shape of the worn surfaces of the 

 teeth. As Mr. Hinton has pointed out to me, these diffei-ences in 

 the form and proportions of the incisors are of impoi'tance as 

 indicating probable differences in the modes of life in these 

 various genera. 



It is interesting to find that an examination of the microscopic 

 structui-e of the enamel in the incisors of Hypnomys seems to 

 bear out the conclusions independently arrived at from a general 

 study of its remains. Mr. Thornton Cai'ter very kindly under- 

 took the task of making this examina,tion, and allows me to 

 include his report of his investigations, which is as follows : — 



" The structure of the enamel of the incisors in the specimens 

 from Menorca and Mallorca is identical with that of Leiihia 

 meliiensis. 



"The 'pattern' is distinctive and presents charactei^s which 

 would seem to place it between Sciuridse and Myoxidee. In 



Proc. Zool. Soc— 1918, No. XVI. 16 



