MICROTINE 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 AZzcro¢znce, so far as is known, to no exception ; thus, 
even the cheek-teeth of Decrostonyx 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 MWiecrotine 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 7° and mz, being particularly diagnostic. 
The arrangement in the various species will be more easily 
understood by a reference to the teeth of Dzcrostonyx, 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 Dacrostonyx (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. 7 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 
