814 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL.SX. 



behind the tooth. External to the posterior part of the ligament there 

 is a submarginal thickening or fold, especially in the right valve. In 

 the left valve the central tooth is represented by an irregular, bilobed, 

 or somewhat V-shaped thickening of the margin, of which the anterior 

 part, situated just in front of the beak, is the more prominent; but this 

 varies in form in different specimens. The postero-dorsal margin along 

 the ligamental region is less thickened but has a distinct rounded 

 ridge inside the ligament. 



Length of one of the largest specimens, 10 mm. ; total height, 10 mm. ; 

 height of cavity, 12 mm.; breadth, 10 mm. In a more rounded speci- 

 men the length is 15 mm.; total height, 10 mm.; height of cavity, 

 13.5 mm.; breadth, 14 mm. 



A few dead specimens of the typical form (sublcvis) have been taken at 

 five stations between N. lat. 39 15', W. long. 08 8', and K lat. 

 37 50' 20", W. long. 70 57' 30", in 1,594 to 1,917 fathoms, 1883-1880. 



Several live and dead specimens of the varietal form (microdonta) have 

 been taken at eight stations between N. lat. 39 26', W. long. 08 3' 30", 

 and N. lat. 30 47', W. long. 73 9' 30", in 1,031 to 1,859 fathoms, 

 1885-1880. 



Mr. Dall extends the range south to Patagonia, in 122 to 1,035 

 fathoms. 



Our specimens show considerable variation in form as well as in the 

 prominence of the cardinal tooth in the right valve, and thus unite the 

 extreme forms P. sublems Yerrill, and P. microdonta Dall. 



CETOCONCHA ATYPHA, new species. 



Shell short-ovate, nearly equilateral, and nearly equally rounded at 

 both ends, judging from the lines of growth, limbos rather prominent, 

 but less so than in several allied species. Beaks rather prominent and 

 curved strongly forward, but not spiral. Surface somewhat shining 

 and slightly iridescent where rubbed, covered with a very thin, yellow- 

 ish epidermis with very numerous, minute, granule-like elevations which 

 are arranged in regular radiating lines, and are much the most numer- 

 ous on the posterior end where the radial rows are closely crowded and 

 the granules in each are also near together; on tlie center the rows and 

 granules are more distant, so that the number is only about half as 

 great in the same space; on the anterior end they are so scattered that 

 the radial rows are indistinct and the granules are a little larger; on 

 the lunular area they are nearly obsolete. The anterior end and lunu- 

 lar area are marked by rather conspicuous lines of growth which, near 

 the dorsal margin, take the form of distinct, raised, concentric ridges. 

 The antero-dorsal margin is nearly horizontal and rises up, in a side 

 view, in an acute edge, a little higher than the level of the beak, so as 

 to produce a broad, compressed, lunular margin. When viewed from 

 above, this part of the margin forms a very marked obtuse angle with 

 the posterior hinge-margin. The postero dorsal margin is also nearly 



