498 MURID^— MURINE 



The principal muscles of mastication are the deep portions 

 of the masseteres laterales ; the temporal muscles are small, and 

 their anterior portions show no tendency to increase in size 

 and strength as in MicrotincB. In the skull, therefore, the 

 lower maxillary roots of the zygomata, in order to accommodate 

 the enlarged masseteres laterales muscles which rise from their 

 outer surfaces, have grown into deep vertical plates of bone 

 which project more or less considerably in advance of the 

 slender upper roots of the zygomata which bridge the infra- 

 orbital canals ; the numerous specialisations, which in the skulls 

 of Microtincs are the outcome of hypsodont cheek-teeth and 

 exceptionally developed temporal and pterygoid muscles, are 

 wholly lacking ; the tympanic bullae are usually small relatively, 

 and of simple structure, being without internal spongy tissue. 

 The slender mandible has usually well-developed coronoid and 

 angular processes. 



The incisor teeth are, in transverse section, deeper than 

 broad, instead of broader than deep as in Microtincs: this 

 difference of shape is an expression of the greater powers of 

 gnawing possessed by Murines, in which these teeth are used 

 as gouges, as compared with Microtince, in which the incisors 

 have the function of cutting or shearing instruments {cf. Ryder, 

 Proc. Acad. Nat. Sci. Philadelphia, 1877, 314). 



The cheek-teeth are brachyodont and rooted ; their crowns 

 are composed in the most primitive forms of three longitudinal 

 rows of tubercles, each row consisting primitively of at least 

 three tubercles. The axis of each tubercle is more or less 

 oblique to the base of the tooth ; in upper molars the tubercles 

 and their grinding surfaces have a general backward inclina- 

 tion, while those of lower molars are inclined forwards. In 

 mastication there is thus little if any longitudinal motion 

 between the upper and lower tooth-rows. The enamel thins 

 out towards the summit of each tubercle and, even in unworn 

 germs, does not cover the dentine at the apex (Hensel, 

 Zeitschr. deutsch. geol. Gesellsch., 1856, 283). 



In order to give a clear view of the meaning of the variations 

 in the structure of the cheek-teeth of the Murines and to 

 facilitate their description, the cusps are numbered or lettered 

 in accordance with the system of notation employed by Hinton. 



