192 MONOGRAPHS OF NORTB AMERICAN KODENTIA. 



Willi what light we have upon the subject at present, we make out the 

 following ease of Richardson's species: — 



1. A. "riparius Ord" apud Rich, (-nriehardsonii And. & Bach.) is a 

 transitional form between true riparius and xanthognalkus, as fully treated of 

 beyond. It lias no peculiarities of incisor dentition. 



2. A. xanthognathus Leach et 'Rich, is positively identified, as explained 

 beyond. 



3. A. " pennsylvanicus Ord'' apud Rich, is a complete synonym of true 

 riparius. 



4. A. " noveboracensis ! Raf.' 1 apud Rich. ( = drummondii Aud. & Bach.) 

 remains indeterminable. It may be a Pedomys. 



5. A. borealis Rich, is a variety of riparius, as treated of beyond. 



6. A rubricatus Rich, is in all probability Mus rutilus Pall. (Evotomys 

 rutilus nobis). 



We are now better prepared to interrogate the specimens before us 

 themselves, to which final appeal must of course be made. At the outset, 

 we lay down two propositions, to be proven in the sequel. 



I. All the specimens before us belong to the riparius section of Arvicola 

 ( — Myonomes). 



II Any characters which may be taken to establish two or more species 

 are found to melt insensibly into those of typical riparius. 



It is only at the close of an unusually protracted and laborious investi- 

 gation that we venture upon this last extreme statement. We had through- 

 out been perfectly satisfied of the specific validity of xanthognathus and 

 borealis. Typical examples of each differ so much from ordinary United 

 States riparius that we could not believe them to be the same, notwithstand- 

 ing all these mice had taught us of the limits of variability in this family. 

 We confidently labeled several hundred characteristic examples of xantho- 

 gnathus, and a large number of specimens of borealis we disposed of with 

 equal readiness. There still remained, however, over a hundred skins, the 

 attempt to identify which upon the supposition of the specific validity of 

 xanthognathus and borealis has involved us in utter confusion, from which 

 we see no hope of escape except through abandoning our former position. 

 It is, we sec now more than ever before, this having to tabulate and label 

 every individual specimen that tells the story and applies the crucial test. 



It is in violation of our preconceived ideas to be obliged to identify such 



