260 A. #. Verrill—Mollusca of the New England Coast. 
side, is regularly elliptical and pretty clearly defined by the ridges 
running from the beak to the posterior tip; this area is covered by 
rather prominent, thin, or somewhat lamelliform, divergent ribs, 
which are regularly and rather closely arranged and somewhat nar- 
rower than their interspaces, distally; these ribs usually cover the 
whole surface, close up to the dorsal edge, which forms a somewhat 
prominent, sharp and nearly straight carina, but is not compressed 
and thin, as in some other related species. Within the dorsal area 
there is no circumscribed area, such as figured by D’Orbigny in Z. 
Jamaicensis, with which Mr. Dall identifies our shell. Moreover, the 
anterior lunule, represented as very distinct in the latter, is obscure 
and often entirely wanting in our shell. When visible at all, the 
lunular area is narrow, elongated and defined only by the interrup- 
tion of the concentric ribs, just before reaching the hinge-margin, 
leaving a nearly smooth dorsal area between the umbos. Moreover, 
on the anterior end of the shell there are two rather faint, slightly 
raised ridges, or waves, a short distance apart, extending from the 
beak to the anterior ventral margin, and having the area between 
them slightly concave, corresponding to a flattened or slightly con- 
cave space on the margin, where it terminates; this slight undula- 
tion, bordered by two small crests, is, however, usually less distinct 
than represented in our figure. The posterior end is decidedly acute 
with a distinct emargination below it, but the ridge, which runs 
from the beak to the posterior tip, though somewhat prominent, is 
obtusely rounded and decidedly less developed than in LZ. Jamai- 
censis. The concentric ribs are clearly defined, usually very regular, 
obtusely rounded, or frequently with the edge reflexed anteriorly, 
and often rising into little angles or points in crossing the posterior 
ridge; the interspaces are rather deep, nearly smooth, and usually 
about twice the breadth of the ribs on the sides of the shell. The 
epidermis is closely adherent and usually dark olive-green. 
Well-grown specimens are frequently 13" long; 8"™ broad; and 
6™™ thick; from the beak to the posterior tip, 8"™. 
A species closely related to this, but evidently distinct, which is 
most likely the true Z. wnca Gould, was dredged by the Albatross 
off Cape Hatteras in 14 to 48 fathoms, in considerable numbers. 
This shell is more solid, more ovate, and more swollen medially, 
with the beak nearly central, the posterior end very acute, and the 
posterior dorsal margin slightly concave, while the concave dorsal 
area is defined by a rounded and not very prominent ridge. The 
umbos are nearly smooth, polished and lustrous in the adult dead 
