Description (from Thorne & Malek, 1968)
Body assuming an open C or U form when relaxed, the tail more arcuate, sometimes hooked. Striae about 1 um apart. Lateral field a plain band, sometimes with 2 minute incisures, about 1/5 body width at midbody. Spear 12 um long with small rounded knobs, the muscles attached to border of cephalic framework and sclerotized cuticular band. Cephalids not observed. Intestine with scattered refractive granules. Ovary outstretched; posterior uterine branch a short pouch. Vulva-anus distance slightly less than tail length. Terminus minutely rounded.
Male similar to female with broad bursa, cephalated spicula and hooked tail.
Filenchus butteus is immediately distinguished by the cylindroid neck, attachment of spear muscles and arcuate body.
Habitat: Native sod from summit of Medicine Butte, S. Dak., and Rugby, N. Dak.