444 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



td invalidate the conclusions that the European and the American Beaver 

 constitute different species. The extremes of difference, in their aggregate, 

 on the one side and on the other, are sufficiently striking to justify us in 

 regarding them as varieties of one and the same species; while the want of 

 constancy in these peculiarities suggests the inference, that these variations 

 are due to long separation of the races, and to accidental causes, rather than to 

 original diversity of the stock. It is conceded by the advocates of a diversity 

 of species that the Beavers of the Old and the New World cannot be dis- 

 tinguished by any external characteristic. The same is true of their habits 

 and instincts, except so far as they have been evidently controlled by external 

 influences. The castoreum secretion is variable, even in the European 

 Beavers, and there are facts to show that the elements of the food of the 

 animal are found in it.* The differences observed in it, being more of degree 

 than of kind, are not of such a character as to render it improbable that they 

 are due to the influence of climate, food, and accidental causes.''f 



A careful analysis of the above-noted cranial differences between the 

 European and American Beavers shows that they consist mainly in (1) the 

 greater general breadth of the anterior portion of the skull, resulting in a 

 greater interorbital breadth, wider nasal bones, wider muzzle, and consequently 

 wider incisors; (2) the relatively greater posterior extension of the nasals; 

 (3) the greater size and depth of the basilar cavity ; and (4) in less marked 

 and rather more inconstant features of difference in a few other points. Con- 

 ceding with Dr. Ely their varietal or subspecific distinctness, the two forms 

 may be thus conveniently diagnosed: 



Castor fiber var. fiber. — Dorsal surface of the interorbital region gener- 

 ally as broad as, or broader than, long ; nasals extending backward beyond the 

 posterior border of the anterior orbital process; basilar cavity deep and large ; 

 bullae placed more anteriorly, etc. 



Castor fiber var. canadensis. — Dorsal surface of the interorbital region 

 generally longer than broad ; nasals generally not reaching beyond the middle 

 of the anterior orbital process; basilar cavity comparatively shallow, etc. 



Synonymy and Nomenclature. — In respect to the distinctive name of the 

 American form, that of canadensis of Kuhl evidently has priority ; the ameri- 



* The castoreum of the American Beaver is well knowu to differ very materially from that of the 

 Old World Beaver, aDd has a very much smaller commercial value. Chemical analyses show that the 

 castoreum of the Russian Beaver coutains more volatile oil, castoriu.and resin, and much less carbonate 

 of lime, than that of the American Beaver. 



t Morgan's " The Beaver and his Works", p. 299. 



