22 MICROTIN-E 



crown appears to be composed, and giving rise to a number of 

 salient angles and re-entrant folds along the inner and outer 

 borders of the tooth. Each triangle or loop is formed by dentine, 

 surrounding an inner core of relatively soft " osteodentine," and 

 bounded externally by a sheet of relatively hard enamel. The 

 front end of each upper molar and the hinder end of each lower 

 molar is formed by a more or less crescentic or pyriform transverse 

 loop, the inner and outer extremities of which form the first inner 

 and first outer salient angles. The triangles behind or in front 

 of this transverse loop are arranged in two series, an inner and an 

 outer, the members of which alternate with each other more or 

 less distinctly and regiilarly. The apex of each triangle forms a 

 salient angle which is separated from its neighbours by a re- 

 entrant fold or cement space, which in many genera is partly 

 filled with cement. At the hinder part of m^ and the fore-part of 

 «rj the triangles are succeeded by a complex structure, the " pos- 

 terior loop " in m^, the " anterior loop " in m^ ; these terminal 

 loops are often of great systematic importance. 



In describing the teeth it is customary to enumerate the 

 salient angles and re-entrant folds from before backwards in 

 upper molars, and from behind forwards in lower molars, the 

 first salient angle on each side being formed by the transverse 

 loop. The differences in the number, form, and relative size of 

 the triangles and salient angles, the degree to which the dentinal 

 spaces are open to or closed off from each other, the greater or 

 less complexity of the anterior loop in m^ and of the posterior 

 loop in w?, the distribution and nature of the enamel sheet in 

 different parts of the periphery, the presence or absence of 

 cement, and above all the circumstance whether the cheek-teeth 

 are of persistent or of limited growth, i.e., rooted or rootless; — all 

 these, when used with discretion, afford excellent characters for 

 the distinction of genera and species. But in using them for 

 such purposes it must always be borne in mind that the pattern 

 is often subject to considerable variation, not only in different 

 individuals, but in different stages of wear in the same individual ; 

 the old idea that " prismatic teeth " present the same pattern 

 throughout life is erroneous. These and many other points are 

 discussed below in the section dealing with the evolution of the 

 teeth (p. 102). 



Jaw Muscles. 



It is not possible to give a full account of the myology of the 

 Microtinse in the present work, but the following notes on the more 

 important muscles of the jaws, based chiefly upon several dis- 

 sections of Arvicola amphibius, will give a fair idea of the general 

 arrangement found in the group. 



The most important muscles are the temporalis and the 

 masseter (Figs. 13 and 14). These have played a great part in 

 moulding the outward form of the skull; the effects of their 



