EVOLUTION OF MOLARS 



109 



(Fig. 586) the valleys do not extend so far back, and so the 

 postero-median tubercle appears to be much larger than in the 



Fia. 58. — Ondatra and Arvicola. 



Crown views of unworn or little worn specimens of the wij. 



a. Ondalra zihdhica. Left m^, unworn. 



b. Arvicola amphibius. Left Wi, unworn. 



c. A. abbotti. Right nij, unworn (Pleistocene). 



d. A. sapidus. Left m^, slightly worn. 



e. A. abbotti. Right Wj, slightly worn (Pleistocene). 



f. A. terrestris. Left Wj, slightly worn {B.M., No. 3.9.25.7). 



g. A. scherman exitus. Right Wj, slightly worn (B.M., No. 5.1L18.23). 

 h. A. abbotti. Left wij, slightly worn (Pleistocene). 



i. A. sapidus. Right m^, slightly worn (B.M., No. 8.2.9.201). 

 j. A. sapidus. Left »»,, slightly worn (B.M., No. 8.9.1.71). 

 k. Ondatra zibtihica. Left mj, slightly worn; drawn to about half 

 scale of other figures {B.3I., No. 94.5.9.39). 



fossil. Each of the crests between the principal valleys, two on 

 each side, is equivalent to a salient angle, so that in the unworn 

 tooth five outer and six inner salient angles are represented ; and 



