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University of California Publications in Zoology [Vol. 21 



manner of the addition of cusps in other groups of mammals make 

 such an interpretation seem improbable, though it must be admitted 

 that at present we do not have definite evidence as to the formation 

 of these loops. It is more probable that the internal portion, at least, 

 of the anterior transverse loop of the fourth upper premolar repre- 

 sents the protoconule, but even in that case its position is very unusual. 



pm, 



m 



pm' 



m' 



m- 



C 



D 



Figs. C, D. Diagrams of enamel patterns of left lower and left upper molari- 

 form series of Microtus caJiformcus calif oriiious. X 7. Mus. Vert. Zool., no. 

 3655, c?; Palo Alto, Santa Clara County, California. 



Explanation: Pni4, fourth premolar; Ml, first molar; M2, second molar; end, 

 entoconid; esd, entostylid; hy., hypocone; hyd, hypoconid; me., metacone; med, 

 metaconid; mesd, mesostylid; ml., metaconule; nits., metastyle; pa., paracone; pad, 

 paraconid; pi., protoconule; pr., protocone; prd, protoconid; prcd, protoconulid ; 

 ps., parastyle; sec. ml., secondary metaconule; sec. pad, secondary paraconid; sec. 

 j)rcd, secondaiy protoconulid. 



In many rodents having a hypsodont type of dentition, the anterior 

 deciduous molar continues its function of slicing and shearing while 

 the permanent molars are coming in. Evidence bearing on this is 

 afforded b}^ the beaver, Castor. The view that this anterior molari- 

 form tooth is a deciduous molar is also held by Forsyth-Major and 

 Martin A. C. Hinton, the latter basing his view upon the extraordinary 

 complexity of the anterior cheek teeth and upon various theoretical 

 considerations. Support is also lent to this view in an instance 

 recorded by Winge of the occurrence of a small fourth posterior cheek 

 tooth in Microtus agrestis. 



If this anterior molariform tooth is the deciduous molar, it would 

 necessarily possess more primitive characters than the permanent 



