MOLLUSCA OF PORTO RICO. 
465 
Pecten medius Lamarck. 
Pecten medius Lamarck, An. sans Vert., VI, 1819, p. 163; Chemnitz, Conch. Cab., vn, pi. I.x, figs. 586, 587, 589, 1784. 
Shell nearly orbicular, rather solid; left valve decidedly concave, with strong, radial shoulders, 
sculptured with well developed, rounded ribs and delicate, somewhat lamellar growth lines, which are 
fainter on the ribs, deeply, radially grooved within at the border; right valve strongly convex, with 
wide, rounded or flattened ribs, which are sometimes broken up into smaller ridges; interior ribs of 
right valve somewhat double; both valves with faint crura at the base of the ears; ears nearly even, 
ridged. Left valve red or purplish sometimes variegated with small, lighter blotches, generally having 
a lighter, well-marked area at the beak; right valve purplish or whitish, sometimes faintly marbled. 
Length, 50; height, 47; diameter, 13 mm. 
Mayaguez Harbor, Porto Rico, two young opposite valves; off St. Thomas, one specimen. 
Subgenus Chi.amys Bolten, 1798. 
Section PLAGIOCTENIUM Dali, 1898. 
Pecten mayaguezensis, n. sp. Plate 55, tigs. 7, 8, 9. 
Shell small, solid, slightly inequilateral, inequivalve; the left or upper valve less convex, polished, 
white, with beautiful zigzag, subconcentric, linear, or flecked painting of dark red; ribs eighteen or 
nineteen, angular at the summit, with subequal, not channeled interspaces, the whole without sculpture 
except microscopic incremental and obsolete fine radial lines; ears practically smooth, the posterior 
longer; lower valve more convex, white except near the umbo, where there are a few brown flecks; 
sculpture about the same, except that the summits of the ribs are more rounded; anterior ear with two 
or three radial threads, a shallow sulcus and short ctenolium; submargins smooth; interior channeled 
near margin with well-marked auricular crura; hinge in right valve strong, with a long, horizontal 
ridge on each side of the resiliary pit corresponding to a similarly placed sulcus in the opposite valve; 
above the last-mentioned sulci in left valve is a similar pair of ridges; hinge showing strong provin- 
cular cross striation; cavity of left valve reddish, of right valve white or yellowish. 
Length of shell, 21; height, 19; diameter, 8.5 mm. Living in 7 to 15 fathoms, sand and mud, 
Mayaguez Harbor, Porto Rico, at stations 6058 and 6061. 
This is an extremely elegant little shell, perfectly distinct from any recorded species. 
Pecten gibbus Linnaeus. 
Ostrea gibba Linnaeus, Syst. Nut., ed. x, p. 698, 1758. 
Pecten gibbus Reeve, Conch. Icon., vm, pi. ix, fig. 37a, 1852. 
Aguadilla, Arecibo; San Juan, Porto Rico (Gundlach). 
Section NODIPECTEN Dali, 1898. 
Pecten nodosus Linnaeus. 
Ostrea nodosa Linnaeus, Syst. Nat., ed. X, p. 697, 1758. 
Pecten nodosus Reeve, Conch. Icon., vm, pi. in, fig. 15, 1852. 
Shell suborbicular, nearly equivalve, sculptured with eight or nine strong, rounded, nodose ribs, 
which are separated by rounded grooves of nearly equal width with the ribs; the entire surface is also 
covered with fine, distinct, riblets and concentric lamellar growth lines, which are faint on the ribs; 
nodules very strong, usually faint or wanting on the upper part of the shell; interior of the valves 
ribbed but otherwise smooth; ears unequal, strongly ridged, the posterior pair smaller; anterior ear of 
right valve deeply notched at its base. Color dark purplish, red, or scarlet, the young shell sometimes 
irregularly concentrically banded with white. 
Length, 75; height, 80 mm. St. Thomas, one young valve. 
Sometimes the nodules become blistered and the outer shell is broken at the blisters. 
Section CHLAMYS s.s. 
Pecten ornatus Lamarck. 
Pecten ornatus Lamarck, An. sans Vert., Vi, p. 176, 1819 (Enc. Method., pi. ccxiv, fig. 5). 
Shell rather thin, high, inequilateral, nearly equivalve, with elevated, smooth, or scaly ribs, which 
are separated by deep, excavated channels; sometimes the ribs are compound, consisting of three or 
more small riblets raised to form a large rib; the interior is ridged to correspond with the external 
sculpture; posterior ears small, slopingly cut away; anterior ears large and projecting, so that the 
whole shell is somewhat oblique; that of the right valve deeply notched, all of them covered with 
F. G. B. 1900—30 
