CLASS PELECYPODA 
33 
Yoldia ensifera plena Dali, 1908. 
Proc. U. S. N. M., 34:256. 
This species in its typical form is abundantly distinguished from the 
Y . scissurata Dali (which is the common Arctic species usually named 
Y. lanceolata in collections), especially by its form and the prominent 
blades upon the posterior dorsal margins, which are three times the size 
of the same parts in the Arctic species. The peculiar grooving of the 
valves in ensifera is comparatively feebler, toward the ends especially, 
compared with that of scissurata, which is not only stronger but covers 
the whole valve except a small portion near the posterior end, while in 
ensifera both ends are usually free from grooves; but Professor Kelsey 
has dredged in 80 fathoms, off the entrance to San Diego Harbor, a 
variety which, while agreeing in form and hinge characters with ensifera, 
has the grooving extending to within 6 mm. of the posterior extremity and 
over the whole anterior end in specimens 28 mm. long, while a large 
series from San Pedro to the Aleutian Islands does not afford a single 
instance of such extension. It seems, therefore, that the difference is 
marked enough to constitute a nameable variety, which may take the name 
of plena. Length, 30; height, 16; diameter, 6 mm. (Dali.) 
Type in U. S. N. M. Type locality, 80 fathoms off entrance to San 
Diego Plarbor. 
Range. Known only from type locality. 
SECTION YOLDIELLA, Verrill, 1897. 
Yoldia oleacina Dali, 1916. 
Proc. U. S. N. M., 52:398. 
Shell small, plump, smooth, except for faint incremental lines, 
brilliantly polished, of a bright yellow brown with a slight olivaceous tint 
toward the beaks; egg-ovate, rounded and broader in front, attenuated 
behind; beaks broadly arcuate, dorsal margin gently curved; beaks low, 
6 mm. from the anterior end; no defined lunule or escutcheon; 13 anterior 
and 11 posterior hinge teeth, the resilifer subumbonal, cup-like; the 
pallial line has a slight shallow sinus near the posterior adductor scar. 
Length, 16; height, 9; diameter, 6 mm. (Dali.) 
Type in U. S. N. M., No. 223407. Type locality, Arctic Ocean north 
of Bering Strait. 
Range. Known only from the type locality. 
Yoldia orcia Dali, 1916. 
Proc. U. S. N. M., 52:399. 
Shell small, thin, pale olive, brilliantly polished, smooth, equilateral; 
base deeply arcuate; beaks not prominent, the posterior end slightly com- 
