838 PROCEEDINGS OF, THE NATIONAL MUSEUM. VOL.XX. 



end, with a sharp, angular, byssal notch beneath it, separated from the 

 body of the shell by a narrow groove. The dorsal margins of the body 

 of the shell are nearly straight and form more than a right angle; the 

 ventral margin is broadly rounded, nearly semicircular, forming a very 

 obtusely rounded angle, where it joins the dorsal margins. Umbos a 

 little prominent, with a small, smooth, rather acute, incurved beak, 

 which projects a little above the hinge-margin. The surface of the left- 

 valve is everywhere thickly covered with fine, almost microscopic, radia- 

 ting stria-, which become a little more distinct on the anterior auricle; 

 slightly raised thin lines of growth are often very distinct on some 

 parts of the shell, especially on the anterior auricle, where they become 

 closer, more regular, and often produce, in crossing the radial striations, 

 a quite regular, microscopic decussatiou; the sculpture on the posterior 

 auricle is sometimes similar but finer, although in many specimens the 

 surface is nearly smooth or marked only by very fine radial striae. The 

 right valve is less convex than the left (its ventral edge does not quite 

 reach that of the opposite valve), the umbo is less prominent, the beak 

 less acute, and scarcely projects beyond, and often falls short of, the 

 hinge-margin; the inequality is less marked than in most .of the allied 

 species. The body of the shell in this valve is smooth, except for very 

 fine, irregular lines of growth; on the anterior auricle there are from 

 three to six, or more, distinct radial ridges, roughened by conspicuous 

 lines of growth; the margin below the byssal notch is entire, without 

 pectinidial teeth; the posterior auricle is nearly smooth. The internal 

 hinge-plate is thin in the middle, but relatively broad on each auricle, 

 and is crossed by numerous fine, well-marked, transverse striations; 

 these are much more conspicuous than in any of the related species, 

 whether young or old. The resilial pit is small, rounded, situated just 

 under the beak. The inner surface is smooth and glossy, although in 

 fresh specimens the external radiating lines show through by transpar- 

 ency. There are no internal line. 



The ground color of the right valve is yellowish or grayish white, 

 with more or less numerous light yellowish brown and reddish brown 

 spots or blotches, and sometimes with irregular patches of opaque white; 

 the right valve is white, sometimes with a few yellowish brown spots. 

 Some specimens are nearly destitute of spots. 



Length of one of the largest specimens, 7 mm.; height, (3 mm.; dor- 

 sal hinge-margin, 4 mm. 



It has been taken in considerable numbers, live and dead, at three 

 stations between N. lat. 37 7' 40", W. long. 74 35' 40", and K lat. 35 

 42', W. long. 74 54' 30", in 43 to 13l> fathoms, 1884. 



Although very small, this species seems to be adult. It is so distinct 

 from all other species of our coast that a detailed comparison is unnec- 

 essary. It resembles the young of /'. cUnton'iis more than any other 

 native species, but a comparison of specimens of the same size shows 

 marked differences. 



