S. transitoria new. Small, 90 by .35, thick and strong; costa finer, regular, interspaces narrow; aperture small margin narrow and rounded, polished; dull reddish brown over all the surface. 250, out of which 15 only were living, and ten of there were young. In the last stage of its specific life. Northern portion of Great Pimlico, ranging almost into the area occupied by the foregoing species; both species living upon the same food and under the same environment.