192 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
With what light we have upon the subject at present, we make out the 
following case of Richardson’s species :— 
1. A. “riparius Ord” apud Rich. (= richardsonit Aud, & Bach.) is a 
transitional form between true riparius and xanthognathus, as fully treated of 
beyond. It has 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? Rat.” 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 r7parius section of Arvicola 
(= Myonomes). 
IL 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 
ranthognathus 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 see 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 
