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