104 MICROTINiE 



with tall-crowned, prismatic cheek-teeth, those teeth possess 

 when quite unworn more or less well-developed tubercular caps, 

 readily explicable as heritages from brachyodont ancestors, but 

 apparently admitting of no other explanation.^ 



The tubercular cap of the unworn tooth is thus of great 

 interest.^ It is found more or less clearly developed on the molars 

 of various hypsodont genera, representing between them all three 

 of the great tribes of Simplicidentate rodents; in all three 

 wherever the cap occurs in a well-developed form it shows the 

 tubercles arranged in three longitudinal rows, a circumstance which 

 by itself would almost suffice to prove that such a triserial arrange- 

 ment, similar to that so familiar in brachyodont Murinse, represents 

 the primitive arrangement of cusps or tubercles in the molars of 

 this order. In certain Microtine genera, e.g., Dicrostonyx and 

 Arvicola, some of the coronal tubercles remain clearly recognizable 

 individually, so that we are able to institute a comparison between 

 them and the tubercles occurring normally in the molars of less 

 specialized Muridse ; but in most Microtines the tubercles have 

 lost their individual distinctness, and such a comparison is there- 

 fore not possible. Speaking generally (if one may be permitted 

 to anticipate the answers to be given to the questions raised on 

 p. 103), in the present subfamily the transverse simplification of 

 the teeth has progressed very far ; so that, although we frequently 

 find traces of some of the tubercles of the median row in this group 

 and evidence that those tubercles in a much modified form build 

 up an important share of the tooth-crown, the triserial arrange- 

 ment is masked to a greater extent than it is in other groups of 

 Muridse, where nevertheless the molars are more reduced longi- 

 tudinally than they are in Microtinse. But when we consider that 

 such wholly ephemeral structures as these coronal tubercles have 

 had no serious functional importance in Microtinse since at least 

 Middle Tertiary times, it is not surprising that they have atrophied 



* A contrary view is expressed by Ameghino in his remarkable 

 " Recherches de Morphologie Phylogenetique sur les molaires superieures 

 des Ongules," Buenos Aires, 1904, pp. 32-36. In this very interesting 

 section of his work at p. 34 he says : — 



" Nous avons done sur les molaires nouvelles dej^ calcifiees mais qui ne 

 sont pas encore sorties de leurs alveoles, des caract4res morphologiques de 

 deux categories d'une signification bien distincte. 



" 1°. Ceux qui sont limites au sommet de la couronne; de ceux-ci, 

 quelques-uns persistent jusqu'^ I'age adulte et sont ceux propres de I'espfece 

 ou du genre, tandis que les autres disparaissent presque immediatement et 

 sont les caracteres precurseurs ou prophetiques destines a acquerir un plus 

 grand developpement et k, devenir persistants chez les successeurs. 



" 2°. Ceux qui distinguent I'ensemble de la molaire, surtout ceux qui 

 se trouvent prfes de la base et du col ; ceux-ci reproduisent a grands traits 

 les caracteres qui etaient propres aux ancetres immediats, mais qui n'existent 

 plus dans I'espfece." That there is truth in this second paragraph will 

 become evident at a later stage (see p. 118). 



^ Good figures of the tubercular cap of the molars of Spalax have been 

 given by Mehely, "Species generis Spalax," pp. 296 and 305, 1913; of 

 He.terogeomys by Merriam, N. Amer. Fauna, No. 8, pp. 84, 85, and PI. 16. 



