2-3 mm

This species, rare and sporadically occurring in America, was found in great numbers at one locality in the Park. In identifying it the writer found almost no information on its ecology or variation, and no ad'equate illustrations. Accordingly, a description of the Yellowstone Park series is given below, together with photographs, and notes on its variation and ecology.
Description: Shell small, discoid, ultradextral; apical surface nearly plane, with the exception of the impressed suture and descending body whorl; umbilical surface concave. Whorls three, regularly increasing in size, elliptical in cross section, the last slightly descending and bearing about twentyfive to thirtyfive ribs, occasionally weak but usually strong; the earlier two whorls generally smooth, but the second sometimes with coarse striae developing into ribs on the body whorl. Periphery bluntly rounded, not carinate, becoming less angular with growth so that the aperture is only slightly elliptic and nearly oval; angulation close to apical sur* face, making the shell seem nearly flat on that side. Umbilicus wide and open, exposing all the whorls, and expanding quite rapidly—much more so than the whorls. H: 1,04; W: 1,11; Diameter 0,81 mm
At the locality in Yellowstone Park Armiger crista is rather variable in its ribbing. A few of the series have only strong striae; more have low, weak ribs. Most of the specimens, however, are distinctly costate, with the number of ribs varying from as few as twentytwo to perhaps forty. One individual (1529B) has the ribs sharply produced at the periphery into semilunar projections, giving the shell a coronate appearance
Ecology: The locality is a small, shallow pond, bordered by a thick growth of reeds. On the submerged dead and decaying reeds at the edge Armiger crista was abundant. Nearly sixty individuals were collected in a short time, and without doubt an hour or two more would have yielded a thousand specimens. Associated with A. crista were Gyraulus similaris (juv.) and Lymnaea palustris nuttalliana.