MOLLUSCA OF PORTO RICO. 
459 
Superfamily ARCACEA. 
Family ARCID4E. 
Subfamily PECTUNCULINA. 
Genus GLYCYMERIS Da Costa, 1778. 
Shell suborbicular, solid, equi valve, subequilateral, convex, with a velvety epidermis; area 
moderate; within porcellanous; hinge wide, flat, curved, bearing divergent, taxodont teeth, which 
.are faint under the beaks; muscular impressions subequal; pallial line simple. 
Glycymeris pennaceus Lamarck. 
Pectunculus pennaceus Lamarck, An. sans Vert., vi, p. 51, 1819. 
Pectunculus pennaceus lteeve, Conch Icon., pi. v, tig-. 24,1843. 
Shell solid, suborbicular, somewhat inflated, with rather full, high beaks which are placed 
slightly in front of the center; ligamental area rather narrow; ligament strong; surface with very low, 
wide, radiating ribs, which, with the spaces between them, are covered with delicate, radiating lirse; 
growth lines fine and varied, forming a peculiarly reticulated or latticed pattern of sculpture; hinge 
line less curved than remaining border of shell; teeth not numerous, faint, or wanting under the beaks 
interior faintly radially striate within the pallial line. The surface is covered with a delicate, velvety 
epidermis, which is often wanting at the umbonal region and soon becomes worn off after the death of 
the animal. Color whitish, with irregular, subradial markings and cloudings of chestnut. 
Length, 60; height, 55; diameter, 35 mm. 
San Juan Harbor; Boqueron Bay, Porto Rico; Culebra; Vieques. 
This species, which is not at all common, was obtained abundantly living and in very fine 
condition at Vieques. Some of these specimens considerably exceed the measurements given. 
Glycymeris pectinatus Gmelin. 
Area pectinata Gmelin, Syst. Nat., p. 1313, 1792. 
Pectunculus charlestonensis Holmes, P, PI. Foss. S. Gar., p. 16, pi. in, fig. 5, 1860. 
Shell somewhat fan-shaped, solid, scarcely inflated, with high beaks and a narrow area, sub- 
equilateral, with about twenty-four rounded, often slightly curved ribs, which are crossed by fine, con- 
centric strise; teeth fainter and sometimes wanting under the beaks; margin of shell decidedly toothed. 
Color whitish, variously blotched with chestnut, the blotches sometimes zigzagged and subconcentric. 
Length, 20; height, 20; diameter, 10 mm. 
Mayaguez, Porto Rico, a few dead valves. 
Subfamily ARCINAt. 
Genus ARCA Linnaeus, 1758. 
Shell generally equivalve, thick, subrhomboid, inflated, ribbed or cancellate, covered with a 
thick epidermis; valves closed below, or open for the passage of a byssus; hinge straight, with a 
nearly or quite continuous row of numerous, almost straight, subradiating teeth; beaks prominent, 
incurved, separated by a lozenge-shaped area, with numerous cartilages embedded in grooves. 
Subgenus Akca s. s. 
Area occidentalis Philippi. 
Area occidentalis Philippi, Abbild. und Besclir., m, p. 14, pi. xvii, B, figs. 4 a-c, 1847. 
Shell oblong, wider and truncated behind, solid, inflated, with a moderate posterior ridge, the 
basal and posterior parts covered with a shaggy epidermis; in front of the middle of the base there is 
a long, narrow gap through which the byssus passes; surface decidedly ribbed; in the middle of the 
shell there are large ribs with three or four smaller ones between; hinge long and straight, with a 
lozenge-shaped central area covered with dark ligament, with resiliary threads embedded in grooves; 
