116 MICROTINiE 



tooth, the dentine being already exposed in each of the dentinal 

 spaces ; therefore no coronal tubercles linger in the m^ of 

 Ondatra. But the form of the prisms is most suggestive, in- 

 dicating that the derivatives of the outer row of tubercles are 

 reduced almost to disappearance, this series contributing no more 

 than the mere tips to the outer salient angles, including that of the 

 posterior loop. The derivatives of the inner tubercles are larger, 

 each forming about half of one of the inner salient angles as in 

 Dicrostonyx. The central or basal portions of the inner salient 

 angles, the greater part of each outer triangle, and the isthmuses 

 connecting the inner and outer triangles are formed evidently 

 by derivatives of the median row. A narrow pass, quite similar 

 to those described in young Arvicola, separates the posterior loop 

 from the first inner triangle ; but all the closed triangles and the 

 anterior loop are already connected with each other by narrow 

 isthmuses of dentine bounded by enamel. Between the posterior 

 and anterior loops there are five alternating and substantially 

 closed triangles. The third outer angle shows a minute prism- 

 fold. The anterior loop is of very great interest, closely resembl- 

 ing in all essentials that of the unworn m^ of Arvicola. It is cleft 

 from before backwards by a median longitudinal valley, which 

 extends from the front margin of the tooth to the isthmus con- 

 necting the anterior loop with the fifth closed triangle. The valley 

 is divided into two portions, an anterior and a posterior, by a little 

 isthmus of enamel and dentine ; its posterior and central portion 

 is confined to the summit of the crown, where it forms a large 

 elliptical enamel-lined lake or islet; the anterior portion is a 

 deep furrow notching the anterior border of the tooth and descend- 

 ing its front surface for a considerable distance. On either side 

 of this median valley there are two salient angles as in Arvicola, 

 but the posterior one on each side is normally formed and per- 

 sists in adult stages of wear instead of being ephemeral as in the 

 latter genus. The anterior salient angle on each side is complicated 

 by traces of a prism-fold and is probably a compound of 

 several originally distinct but now highly reduced and ephemeral 

 elements. 



In Fig. bSk a slightly worn m^ of Ondatra is shown. In this 

 tooth also we see distinct traces of some of the ephemeral parts 

 just described ; the last trace of the median valley and remnants 

 of the other complications of the anterior loop are particularly 

 noteworthy. 



It would be tedious to describe the young specimens of m^ 

 for every genus in which I have observed ephemeral complications ; 

 such features occur normally in the young of many widely separated 

 genera, e.g., Synaptomys, Dolomys, some species of Evotomys and 

 Microtus, and in the figures accompanying this work various 

 examples are shown to which it is unnecessary to make any special 

 reference. Among voles Prometheomys and Ellobius have the 

 m^ simpler, in a longitudinal sense, in adult stages of wear than in 



