EVOLUTION OF MOLARS 



117 



other genera ; in these two genera the molars (Figs. 55 and 56) 

 retain many primitive characters, e.g., extreme brachyodonty (for 

 Microtinaj), confluent dentinal spaces, U-shaped re-entrant folds, 

 undifferentiated enamel, and absence of cement; therefore the 

 simplicity of m^ might not unnaturally be regarded as a primitive 

 feature too. But the unworn ?«i of Promeiheomys (Fig. 62) 

 is a relatively complex tooth with a large anterior loop, which 

 is sharply de-limited from the fourth and fifth triangles behind by 

 the well-developed third outer and fourth inner infolds ; the cap of 

 the loop is tubercular and shows traces of an islet and a groove, 

 similar to, though far weaker than, those present in the unworn 

 Wj of Ondatra. In the posterior loop cusp x is large and 

 distinct; in jMo, m^ and ?«^ not only x but the other median 

 ubercles are consj^icuous features of the unworn crown. In 



f 5 ' f 5 



Fig. 62. — Unworn molars of Prometheomys schaposchriikou'i Satunin. 



Crown views : a. right m^ and m\ b. left ?nj and m^ arranged and 

 lettered to show the homologies of the cusps. 



Prometheomys m^, when unworn, is almost as complex as that of 

 Arvicola and owes its adult simplicity to a process of reduction. 

 In Lemmus and its allies m^ is similarly simple ; but in these forms 

 the simplification is of such ancient standing and the teeth are 

 in other ways so highly specialized that unworn examples of the 

 »ij no longer show any trace of a greater complexity in front, and 

 only some traces of the three primitive rows of tubercles. 



Not much need be said about the unworn crowns of the 

 remaining cheek-teeth. In general they show similar traces of 

 the three longitudinal rows of tubercles, and these traces indicate 

 that in each tooth a great part of the crown is derived from the 

 median row. 



The homologies of the cusps are most easily seen on comparing 

 a left m^ with a right m-^ or vice versa^ ; the posterior loops of all 



' Drawings of upper and lower molars are arranged and lettered for 

 such a comparison in Figs. 02, 03 and 77. 



