The Collaborative International Dictionary of English v.0.48:

Sewellel \Se*wel"lel\, n. [Of American Indian origin.] (Zool.)
   A peculiar gregarious burrowing rodent (Haplodon rufus),
   native of the coast region of the Northwestern United States.
   It somewhat resembles a muskrat or marmot, but has only a
   rudimentary tail. Its head is broad, its eyes are small and
   its fur is brownish above, gray beneath. It constitutes the
   family Haplodontidae. Called also boomer, showt'l, and
   mountain beaver.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sewellel
    n 1: bulky nocturnal burrowing rodent of uplands of the Pacific
         coast of North America; the most primitive living rodent
         [syn: mountain beaver, sewellel, Aplodontia rufa]