A. E. Verrill—Mollusca of the New England Coast. 443 
and strongly produced backward; the posterior margin is very 
broadly rounded, its direction nearly parallel to the opposite part of 
the anterior margin. The edge of the shell is crenulated with a row 
of small rounded tubercles situated just within the margin, the larg- 
est along the ventral edge, disappearing toward the anterior end. 
Small radial grooves run inward from between these tubercles for a 
short distance. The ligament-area is unusually wide, somewhat con- 
cave, with a large cartilage-pit in the middle, which is elongated in 
a direction transverse to the ligament, with the sides parallel and the 
apex triangular. The hinge-plate is thin in the middle, becoming 
rather broad at each end, so that the inner margin is curved or angu- 
lated in the middle. The anterior end bears about four prominent 
rounded teeth, the outermost the largest. The posterior end has 
four or five prominent teeth, increasing in size outwardly; the last 
two are decidedly larger than the rest and somewhat oblique. The 
umbos are rather prominent and the beak curves directly imward 
towards the cartilage-pit, and is situated some distance from the 
margin, owing to the breadth of the ligamental area. The surface is 
covered with small, rather regular concentric undulations or ridges, 
which are crossed by radiating lines that are not very distinct over 
the greater part of the shell, and become nearly or quite obsolete on 
the umbos. The epidermis is light yellowish brown, and rises into 
series of slender hair-like processes along the radiating lines; these 
epidermal hairs become longer and crowded toward the margin, 
where they are more or less united and form a marginal fringe. 
Length, 10°5"™; height, 11" ; thickness, 8™™; length of dorsal 
margin, 5™"; breadth of ligament-area, 2™™. 
Station 2092, in 197 fathoms; two living specimens (No. 44,829.) 
This species resembles Z. minuta in size and general appearance, 
but it is more oblique and more produced ventrally, and is widely 
different from that and all our other species, except Z. plana, in hav- 
ing a broad ligamental area and large cartilage-pit. It is also pecu- 
liar in the character of its hinge-margin, and in its teeth, which are 
few in number, prominent, rounded, and scarcely oblique, except the 
outer ones on the posterior side. Externally the surface is smoother 
than in most species, the radial lines being but little evident when 
the epidermis is off. Although resembling Z. plana in its broad 
ligament-area, it differs in having the ventral margin strongly crenu- 
lated, instead of plain, and in form it is a narrower, more oblique, 
and more swollen shell. 
