406 ; [OcToBER, 
Hair above dark plumbeous tipt with brown and black, beneath dark slate- 
colored mixed with brown, particularly on the breast, the upper and.under sides 
of the body being nearly alike. Head large and blunt; ears rounded, longer than 
the fur, outwardly hairy, inwardly only so on the upper margin; antitragus large, 
semicircular; whiskers black and grey, shorter than the head. Feet covered with 
shining grey ‘hair ; thumb tubercle with a compressed, sharp, hooked nail. ‘Tail 
slender, thinly covered with short hair, above mixed brown and black, beneath 
brownish eve 
Length in. ; head 1-3; ears °8; fore leg 1-1; hind leg 1:5; tail 1:4. 
4. Arvicota Riparius. Brevis et robustus, antice quam postice latior. Supra 
fuscus, per dorsum subnigricans, subtus cinereus. Capite obtuso, auribus sub 
pilis occultis, antitrago magno. Cauda pilis brevibus bene vestita, supra nigra, 
subtus pallidissime fusca. 
Hab. In paludibus provinciarum borealium. (Phila., Mr. Ord.) <A. riparius 
Ord., Journ. Acad. Nat. Sci., vol. iv. p. 305. A. pennsylvanica. Richardson, 
Fauna boreali americana, vol. i. p. 120. Idem Audubon and Bachman Quad- 
rupeds of N. America, vol. i. p. 341. Id. Wagner’s Suppl., vol. ii. p. 588. Id. 
Schinz, vol ii. p. 247. A. hirsutus Dekay, Zoology of New York, vol. i. p. 86. 
Body short and robust, broader before than behind. Hair dark lead color, 
above tipt with brown, darker along the back, beneath with grey. Head short, 
larger and blunter than in any other species; ears short, rounded, hairy both 
within and without, entirely concealed under the fur ; antitragus large, semicir- 
cular, nearly closing the auditory aperture; legs and feet brown, the latter 
covered with short, shining hair; thumb tubercle, with a short compressed sharp 
nail. Tail well covered with hair, above dusky, beneath very pale brown. 
Length 6 inches; head 1-4; breadth of head 1°8; ears .4; fore leg 1 in.; hind 
leg 1:8; tail 1:3. 
Swims and dives well; is found peculiarly on the marshes of those rivers 
where the Zizania aquatica grows. As there is a considerable difference in the 
teeth of some of these animals, I have, whenever it was in my power, pointed 
out their peculiarities. With the exception of one species which will be men- 
tioned hereafter, they differ much from those of Europe. Unfortunately I have 
not been able to obtain crania of many of those here described, otherwise certain 
distinctive marks might have been obtained, wala would have rendered their 
determination much more easy. 
System of Dentition.—In the upper jaw the fret molar is composed of 56 tri- 
angles, 1 anterior, 2 exterior and 2 interior; the second of 4 triangles, 1 anterior, 
2 exterior and 1 interior, with a small posterior, rounded lobe ; the third of 5 tri- 
angles, 1 anterior, 2 exterior very small, 2 interior, and ends in a treffle, the ex- 
terior lobe of which is nearly obliterated, leaving a plane space from the end of 
the tooth to the posterior triangle. 
From this arrangement it results, that the line of teeth in the upper jaw con- 
sists externally of 9 salient and 9 re-entering angles, the two last of each small, 
and internally of 10 salient and 8 re-entering angles, exclusive of the interior 
emargination of the treffle. 
In the lower jaw the first molar commences with a treffle, then follow 6 tri- 
angles, 2 exterior, 3 interior and 1 posterior; thé second of 5 triangles, 2 exterior, 
2 interior and 1 posterior; the third of 3 triangles, 1 anterior, 1 intermediate 
and 1 posterior; and the line of teeth is formed externally of 11 salient angles, 
including the exterior lobe of the treffle, and 9 re-entering angles, including the 
concavity of thé side of the treffle, and internally of 11 salient and 11 re-entering 
angles, including the lobe and the emargination of the trefile. 
5. Arvicota Onema. Supra niger, pilis sparsis fuscis intermixius, pluribus 
Super caput et ad genas, fereninee et abdomine saturate plumbeis, hujus pi- 
lorum extremitatibus fuscis, illorum pallidioribus et cinereis. Auribus subpilis 
occultis. Cauda supra nigra, subtus nigrante. 
Hab. A Noveboraco ad Pennsylvaniam. Arv. Oneida Dekay, Zoology of 
New York, l. c. p. 88. 
Hair short and close, not shining; on the back black, with scattering brown 
