Distinctive features:

+ Posterior end distinctly more pointed than anterior end
+ Posterior end obliquely truncated, or pointed, or both
+ Length of hinge more than 0.63 of total length
+ Posterior dorsal margin forms a straight line
+ Posterior end of shell bluntly truncated

Type localities. Y. lucida, northern Norway, Hammerfest, here designated; Leda obesa, Massachusetts Bay, deep water* [maximum about 200 m]; Y. iris, northeastern U.S.A., off Marthas Vineyard, 433 m; Y. subangulata, Maine, 93 m.

Distribution. Both sides of the North Atlantic in depths between 100 and 1000 m, occasionally more shallow. The southern limits are difficult to ascertain from the literature because of confusion with other species. I have, however, examined shells from the Mediterranean and can confirm the reports on its occurrence there (for instance Di GERONIMO & PANETTA 1973).