Macoma oneilli: Shell small, thin, oval, compressed, inequilateral, slightly inequivalve, the anterior part more than twice as long as the part behind the umbones; white covered with a dehiscent yellowish-olive periostracum; beaks hardly perceptible, ligament prominent, posterior end with an extremely narrow compressed dorsal margin corresponding to the fold of most species; anterior and posterior ends subequally rounded; dorsal and basal margins nearly equally arcuate; interior chalky white, the hinge with two small but well-marked teeth in each valve, the anterior left cardinal bifid; pallial sinus rounded behind, coalescent below, a little smaller in the right valve than in the other; muscular impressions subequal and of moderate size. Length of shell 21; length in front of the vertical from the beaks 15; height 15; diameter 6 mm.

Macoma oneilli: This species is remarkable for its evenly oval shape and flattened valves, devoid of the usual posterior twist. Among the various species of Macoma in the Arctic seas it approaches nearest to M. moesta Deshayes, but is quite distinct.

Macoma oneilli: Canada. Dredged in Dolphin and Union Strait in 30 to 50 fathoms, mud and pebbles.