MIMOMYS 371 



with those of Arvicola in pattern (Fig. 102) ; wij has a posterior 

 loop, followed by three substantially closed triangles, and is 

 completed by an anterior loop of complex structure ; there are 

 three outer and five inner salient angles, two outer and four inner 

 re-entrant folds; the third outer salient angle is simple and not 

 compUcated (in adults) by a prism-fold ; the first outer infold may 

 be rather shallow leaving the first pair of triangles more or less 

 confluent with each other ; j/^g and Wg are of the normal Micro- 

 tine form ; the first outer fold in mg is sometimes shallow, leaving 

 the first and second triangles partly confluent; the third outer 

 salient angle is always well developed in m^. 



It is interesting to trace the steps by which the adult simplified 

 pattern of ?«i is developed gradually from the tubercular cap of 

 the unworn tooth. Although several hundreds of specimens of 

 this tooth have been carefully examined, not one has been found 

 to show the tooth in a quite unworn condition ; such a specimen 

 would be of the greatest interest. The earliest stage available is 

 afforded by a young left iiii in Mr. Savin's private collection 

 from West Runton. In this tooth (Fig. 99, r) the dentinal spaces 

 are all widely confluent with each other ; the pulp cavities and the 

 cement spaces are widely open below, and the tooth would obviously 

 have to grow for a long time before closure of the cement spaces 

 could begin. At its present surface the third outer angle bears a 

 weak " prism-fold " (the anterior costa of this fold being hardly 

 indicated), which dies out rapidly when traced downwards upon 

 the outer side of the crown. In front of it is a wide and deep 

 (transversely) third outer valley containing cement; this also 

 dies out below. Other elements of the region corresponding to 

 the adult anterior loop are a large fifth inner angle, a small fourth 

 outer angle and a small sixth inner angle ; the two last-named 

 structures are at present unworn tubercles, separated from each 

 other externally by a shallow oblique sulcus representing a reduced 

 fourth outer valley ; another shallow sulcus, the fifth inner 

 valley, separates the tubercular sixth inner angle from the large 

 fifth inner angle. All these unusual features would vanish quickly 

 with further wear, and, as the open pulp cavities below show, this 

 tooth, in adult life, would have become precisely like any other 

 normal example of the m^ of M. i)itermedius. A right m^ (Fig. 

 99, s), also in Mr. Savin's collection, is slightly more worn; the 

 weak prism-fold and intact third outer valley are still to be seen ; 

 the tubercles representing the fourth outer and sixth inner angles 

 have been worn away and those structures are now blended in 

 the anterior loop. In the left m^ described above all trace of 

 a fourth outer valley would have been lost had it suffered as much 

 detrition as the present tooth has done ; but in this there is in 

 front of the third outer valley upon the outer side of the crown a 

 persistent vertical groove, which on the grinding surface appears 

 as a small curl in the enamel of the outer border of the anterior 

 loop ; this groove is probably to be regarded as a weak vestige of 



