MICROTIN^ 387 



obliteration of one of the three longitudinal rows of tubercles, 

 but, in addition, as insisted by Hinton, certain tubercles in the 

 anterior and posterior portions of each tooth have undergone 

 reduction, though to a very variable extent in the different 

 genera. This phenomenon is the expression of a general law, 

 subject in Microtince, so far as is known, to no exception ; thus, 

 even the cheek-teeth of Dicrostonyx and Fiber, the most complex 

 known in the group, show, when unworn, ephemeral traces of 

 further longitudinal complications which may have been 

 inherited from a remote ancestor. 



From a systematic point of view the enamel pattern of the 

 cheek-teeth of Microtincs is of great importance. Although 

 abnormalities are found in 25 per cent, of individuals of some 

 species, these are frequently appearances caused by different 

 stages of wear. The pattern is, on the whole, remarkably 

 characteristic and constant in each species or group of species, 

 certain teeth, especially m^ and nti, being particularly diagnostic. 

 The arrangement in the various species will be more easily 

 understood by a reference to the teeth of Dicrostonyx, in which 

 the pattern is seen in its longitudinally most complex and 

 primitive form — a form from which all the other patterns may 

 be derived by the reduction or fusion of one or more elements. 

 For convenience sake the parts of each upper tooth are 

 enumerated from before backwards, those of each lower tooth 

 from behind forwards. The prisms are spoken of as " triangles " 

 or " loops " according to their form in the crown view of the 

 teeth. In every case where suitable material could be 

 examined, Hinton finds that "loops" are combinations of two 

 or more cusps. 



In Dicrostonyx (Fig. 55) m} is composed of an anterior 

 transverse loop, followed by six alternating triangles, the anterior 

 internal ; the postero-internal is somewhat reduced in size, the 

 postero-external is vestigial ; briefly described, this tooth has 

 seven dentinal spaces, and presents on each side four salient 

 angles separated from each other by three infolds, m^ pos- 

 sesses a similar pattern, but is more reduced, having only 

 two instead of three inner triangles following the anterior loop ; 

 the anterior is outer instead of inner, and as a whole, this tooth 

 possesses six dentinal spaces, four outer salient angles with 



