329 



largest and the smajlest, excluding the most extreme examples, is one- 

 sixth of the dimensions of the smaller and one-seventh of the size of 

 the larger. 



The sexes differ considerably in size, relatively about the same as in 

 Putorius vison-j but the above generalizations are based wholly on males, 

 and in each case on those of practically the same age, only specimens 

 indicating mature or advanced age being used. 



The series of fully one hundred skulls of this species contained in the 

 National Museum presents a considerable range of variation in details 

 of structure, involving the general form of the skull, the relative size of 

 different parts, and the dentition, especially the form and relative size of 

 the last molar. In a former paper,*. I had occasion to notice somewhat 

 in detail the variations in color our American Martens present, and 

 the difficulty of finding any features of coloration that seemed to indi- 

 cate more than a single American species, or that would serve to 

 distinguish this even from the Martens of the Old World. Dr. J. E. 

 Gray, it is true, had already called attention to the small size of the 

 last molar in the American Martens as compared with the size of 

 the same tooth in the Old World Martens ; but, as his observation was 

 apparently based on a single American skull, and as I was at the time 

 strongly impressed with the wide range of individual variation I had 

 found in allied groups, even in dental characters, and also with the 

 great frequency of Dr. Gray's characters failing to be distinctive, I was 

 misled into supposing all the Martens might belong to a single circum- 

 polar species, with several more or less strongly-marked geographical 

 races. My friend Dr. Coues some months since kindly called my atten- 

 tion to the validity of Dr. Gray's alleged difference in respect to the 

 size and form of the last molar, which I have since had opportunity of 

 testing. This character alone, however, fails to distinguish Mustela foina 

 from Mustela americana, in which the last molar is alike, or so nearly so 

 that it fails to 'furnish distinctive differences. .The size and general form 

 of the skull in the two are also the same, the shape of the skull and the 

 form of the last upper molar failing to be diagnostic. The second lower 

 true molar, however, in Mustela foina presents a character (shared by all 

 the Old World Martens) which serves to distinguish it from Mustela ameri- 

 cana, namely, the presence of an inner cusp not found in the latter. In 

 Mustela flavigula, the last molar is relatively smaller than even in Mus- 

 tela americana, and of the same form. Mustela martes differs in its more 

 massive dentition and in the heavier structure of the skull, but espe- 

 cially in the large size of the last molar and the very great development 

 of its inner portion. Hence, while the size and shape of the last upper 

 molar serves to distinguish Mustela martes from Mustela americana, it 

 fails as a valid distinction between Mustela americana and Mustela 

 flavigula and Mustela foina. As already remarked, however, Mustela 

 americana lacks the inner cusp of the second lower molar, which is pres^ 

 ent in the Old World Martens, or at least possesses it only in a very 

 rudimentary condition. 



* " Mammals of Massachusetts", Bull. Mus. Coinp. Zool., vol. i, pp. 161-167, Oct., 186i). 



