From Lewis and Clark's account of the sewellel, Rafinesque (1817), 
named the animal Anisonyx (2?) rufa. His account reads: 
"Anisonyx ? rufa Raf. Fur long, silky, entirely reddish 
brown, ears short, pointed with short hair =- Obs. This animal 
called Sewellel by Capts. Lewis and Clark, is of a doubtful 
genus, since they only saw the fur of it; it burrows and runs 
on trees like the ground-squirrels; length eighteen inches, 
found in the neighborhood of the Columbia River." 
Rafinesque's Anisonyx brachiura was based upon Lewis and Clark's 
burrowing squirrel and Anisonyx rufa upon their sewellel. Also, on the 
same page, Rafinesque names Lewis and Clark's "barking squirrel," 
Cynomys ludovicianus, now known as the prairie dog. The genus Anisonyx 
referred to Lewis and Clark's "burrowing squirrel," since ascertained by 
taxonomists to be another species of prairie dog (Cynomys columbianus); 
hence a synonym for Anisonyx brachiura, 
Due to the misunderstanding by Rafinesque of Lewis and Clark's mean- 
ing of these two mammals, Harlan (1825) transferred the name sewellel 
(Anisonyx rufa) to the genus Arctomys, but retained the specific name 
rufa because it applied to the sewellel's external description. However, 
Coues (1876) pointed out that naturalists constantly adopted and retained 
scientific names given upon a known basis, even when such names were un- 
qualified by diagnosis. Therefore, he reasoned it invalid for Harlan to 
change the generic designation of sewellel and continued to use sewellel 
for the mountain beaver, 
In 1829, Sir John Richardson became the first naturalist to study a 
mountain beaver skin. His description was based on specimens collected 
by David Douglas, from which he characterized Aplodontia leporina. 
™, Rafinesque has given few details on the genus Anisonyx, 
and says of the teeth, merely, that they resemble those of 
the squirrels. This character does not apply at all to the 
Sewellel, which was known to Rafinesque only from Lewis and 
Clark's description. I have, therefore, been induced to 
constitute a new genus for the reception of this animal, and 
as the specific name of rufa is quite inapplicable, I have 
given it one derived from the general resemblance it bears 
to animals of the hare kind." 
From dentition, examination of the mandible, cranial measurement 
differences, and white interspersed hairs in another specimen, Richardson 
alluded to the possible existence of a second species. He wrote the 
following accounts: 
"The specimen is not sufficiently perfect to enable me to 
give it characters as a distinct species, but I have little 
doubt of its being so." 
The existence of Richardson's second species has never been verified or 
disproved. 
