Halcampoididae with the proximal end rounded. Body cylindrical of considerable length, not divisible into regions, smooth without tenaculi. No sphincter. Tentacles short, conical 40 in number, not bulbously swollen at the apex, the inner tentacles longer than the outer ones. A single, ventral siphonoglyph. Pairs of mesenteries 20 (10+10) all perfect and fertile. Retractors diffuse, band-like. Parietal muscles slightly differentiated. Cnidom: spirocysts (long), basitrichs, microbasic p-mastigophors.
S. laevis Carlgren 1921
Definitions and contained taxa are those of Oscar Carlgren as published in 1949 : Kungl Svenska Vetenskapsakadamiens Handlingar, Series 4, Volume 1, Number 1