362 MICROTIN^ 



tively slender, supporting the first closed triangle (second inner 

 prism) in m^, and the inner part of the anterior loop in m^. 

 Enamel pattern of these two teeth normal, m^ with two roots. 

 Young specimens of this tooth (Mehely, op. cit., Taf. iii, fig. 3) 

 show an anterior loop, followed by four alternating triangles, 

 of which the last two are more or less confluent with each other ; 

 on each side of the tooth there are three well-developed salient 

 angles, separated by two deeply re-entrant folds. In older 

 stages, as the tooth wears down, the central portion of the 

 postero-internal re-entrant fold is seen to lose its connection 

 with the periphery of the tooth in the deeper levels of the crown, 

 and is converted into an enamel islet, which long persists as a 

 conspicuous and highly characteristic feature of the worn surface 

 of the adult tooth. In such teeth there are three outer but only 

 two inner salient angles, and correspondingly two outer but only 

 one inner re-entrant fold. But on the inner side of the crown, 

 opposite the enamel islet, a long persistent though shallow vertical 

 groove remains as the peripheral vestige of the reduced second 

 inner fold (Fig. 99, 22, 22a). Disregarding the islet and the 

 external vestige of the second inner fold (which are vanishing 

 elements), the adult m^ of M. plioccBnicus is an extraordinarily 

 simple tooth showing that this species in spite of its many very 

 primitive characters was highly and perhaps precociously special- 

 ized in this one direction. 



Lower cheek-teeth (Figs. 99, 100). — All three are provided 

 when adult with two roots each, m^ has its crown composed 

 of a posterior loop, followed by three substantially closed 

 triangles, and completed by an anterior loop of complex structure. 

 This anterior loop is composed of three chief elements, viz., the 

 fourth and fifth triangles (forming the third outer and fourth 

 inner salient angles respectively), and a small anterior loop 

 proper, which is produced internally to form the fifth inner salient 

 angle. The third outer angle (or fourth triangle) is partially 

 cleft from its worn surface to the base of the crown by a vertical 

 furrow called by me the " prism-fold " ; each lip of the prism- 

 fold forms a small salient angle. In the type, from the Val 

 d'Arno, the prism-fold is narrow and deep, but in specimens 

 from England and Hungary it is usually wider and shallower. 

 As a rule the prism-fold does not contain cement; but in one 

 tooth a little was observed in this situation (Fig. 100, 4). In young 

 examples (including such as are commencing to develop their 

 roots. Fig. 99, 1, 2), there is a deep re-entrant fold — the third 

 outer valley — immediately in front of the third outer angle; 

 this fold is partly filled with cement. In the deeper parts of 

 the crown, exposed at the surface in later stages of wear, this 

 fold (hke the second inner fold of nfi) is reduced; its internal 

 portion is cut off from the periphery of the tooth and converted 

 into an enamel islet filled with cement ; its outer portion persists 

 as a wide shallow vertical groove channelling the outer surface 



