Shell small, white, inflated, slightly inequivalve and inequilateral, sub-rostrate posteriorly. The right valve a little the smaller ; the lateral outline of the valves viewed from within recalls Poromya granulata, but the shell is much more inflated ; granulations faint or obsolete except behind, as in the last species ; teeth of the hinge obsolete in the adult; anterior and basal margins rounded ; behind is a slight concave wave in the margin below, while the upper posterior margin descends more rapidly than the anterior one, and is subtruncate, and the rostration thus produced gapes slightly; the beaks are inflated, but do not rise very high above the hinge-line. Max. lon. 7.3; alt. 5.5 ; diam. 6.6 mm.
Soft parts as in the last species, but the retractile siphon much shorter ; the respiratory laminae nine on each side anteriorly, the posterior series about eight, and the appendix with about six lamellae. The lower palpi present, small ; the foot slender, grooved behind, and the other features as in the description of the subgenus.
Habitat. Station 44, off Tortugas, in 539 fms.; Station 221 x near Santa Lucia, in 423 fms., ooze ; and Station 176, off Dominica, in 391 fms., ooze; living at all the stations; the bottom temperature ranged from 39°.5 to 43.°5 F. The fragility of the shell is such that nearly all the specimens were broken in the trawl.
This little species was overlooked in my preliminary examination and taken for the young of the preceding. A more careful study shows they are absolutely distinct.