498 MURIDAE—MURIN/Z 
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 MMZtcrotine. In the skull, therefore, the 
lower maxillary roots of the zygomata, in order to accommodate 
the enlarged masseteres /aterales 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 Microtine are the outcome of hypsodont cheek-teeth and 
exceptionally developed temporal and pterygoid muscles, are 
wholly lacking ; the tympanic bulle 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 J/icrotine: this 
difference of shape is an expression of the greater powers of 
gnawing possessed by A/usznez, in which these teeth are used 
as gouges, as compared with MMZzcrotzne, in which the incisors 
have the function of cutting or shearing instruments (cf Ryder, 
Proc. Acad. Nat. Sct. 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 A/urine and to 
facilitate their description, the cusps are numbered or lettered 
in accordance with the system of notation employed by Hinton. 
a — 
