790 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL.XX. 



Four specimens, at three stations, off Marthas Vineyard, in 100 to 

 390 fathoms, 1880-1885. 



AXINULUS, new subgenus or genus 



Type. Axinulus brevis, new species. 



We propose this division to include those species which agree with 

 Cryptodon in the character of the hinge and ligament, but lack the 

 plications of the shell, and have, therefore, a smaller posterior adductor 

 muscle. 



CRYPTODON (AXINULUS) BREVIS, new species 



(Plate LXXXIX, figs. 7, 8.) 



Shell small, short, the height exceeding the length, somewhat pyri- 

 form, with slightly prominent urnbos and small subceutral beaks, which 

 are but little prominent and turn forward. The aiitero- and postero- 

 dorsal margins are about equal in length, the latter slightly more 

 broadly rounded than the former, which is a little incurved near the 

 beak so as to form a very slight lunular area; both ends are broadly 

 rounded and nearly equal; the ventral margin is slightly convex and a 

 little produced just in front of the middle; an exceedingly faint, 

 scarcely discernible undulation runs from the beak to the posterior 

 ventral margin. The surface is covered with very fine, close, parallel 

 lines of growth visible only when much magnified. Grains of fine fer- 

 ruginous sand or mud usually adhere closely to the surface, both ante- 

 riorly and posteriorly. The posterior ligament is well-developed and 

 occupies a well-marked marginal groove: a small, thickened, more 

 internal portion, situated just behind the beak, within the margin, 

 appears to be continuous with the external ligament. The inner edge 

 of the dorsal margin is slightly thickened, for a short distance, just in 

 front of the beak. 



Length, 2^ mm.; height, 2.5 mm. 



Several live specimens and separate valves were found at six sta- 

 tions, between K lat. 40 16' 50", W. long. 67 5' 15", and K lat. 38 I 

 22', W. long. 70 17' 30", in 984 to 1,825 fathoms, 1883-1880. 



At station 2208 was found a single imperfect valve closely resem- 

 bling this species but of much larger size. 



Length, 5.5 mm.; height, 6.5 mm. 



It is, however, much less regular in outline, having a nearly straight, 

 rapidly sloping antero dorsal margin, merging very abruptly into the 

 broadly and very slightly curved and sloping anterior margin, forming 

 a somewhat angular and little produced anterior end; ventral margin 

 strongly convex, curving gradually into the posterior margin which 

 slopes rapidly from the beak; postero-dorsal margin is convex but 

 rises only a little above the outline of the distinct ridge which borders 

 the ligameutal area. 



