792 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL.XX. 



before and behind the beaks, but there is no tooth-like prominence at 

 any point. 



Length, about 3.1 mm. ; height, about 3 mm. 



One imperfect specimen, station 1093, X. lat. 39 56', W. long. 69 

 45', in 349 fathoms, 1882. 



This species is remarkable for the plainness of its surface, and the 

 simplicity of its hinge, as it has neither radial undulations nor tooth- 

 like projections on the hinge-margin. In form it greatly resembles 

 Axinopsis orbiculata, but lacks the conspicuous concavity in the 

 antero-dorsal margin. It has, however, a very obvious posterior liga- 

 mental furrow in the same relative position as that of other species of 

 Cryptodon. 



A single valve taken at Eastport, Maine, 1872, agrees closely with 

 the type in form, but is somewhat less thin and hyaline and the beaks 

 are a trifle more prominent. The surface has faint and rather distant 

 concentric undulations, visible only under the microscope, being most 

 distinct on the uinbo. The microscopic striations are a little more dis- 

 tinct and in some lights give to the surface a fibrous or finely verinic- 

 ulate appearance when highly magnified. This character, however, 

 has been noticed in other species. The hinge-margin is a little more 

 thickened and has a minute swelling on the inner margin just beneath 

 the beak, scarcely worthy the name of tooth; the ligameutal groove is 

 also somewhat more strongly marked. This may prove to be a distinct 

 species more nearly related to Axinopsis orbiculata from which it differs 

 in having the antero-dorsal margin convex instead of strongly concave, 

 and the general outline more evenly rounded, and a less evident tooth- 

 like thickening of the hinge-margin. 



Length, about 2.8 mm. ; height, about 2.6 ram. 



CRYPTODON (AXINULUS) PYGM^US, new species. 

 (Plate LXXXVI, figs. 3, 4.) 



Shell minute, somewhat compressed, transversely ovate, inequi- 

 lateral, with the anterior end the longer, and with a slightly produced 

 posterior augulatiou. Surface scarcely lustrous, covered with fine 

 lines of growth and microscopic striations, and more or less incrusted 

 with ferruginous mud, especially posteriorly; there is barely a trace of 

 a posterior fold. TJinbos a little prominent, beaks small, slightly 

 raised above the margin, and turned a little forward. The antero- 

 dorsal margin is nearly straight, or sometimes slightly convex, with a 

 slightly excavated, small, lunular area; the anterior end is broad, con- 

 siderably produced, and evenly rounded; the ventral margin is broadly 

 rounded, not at all produced, and joins the posterior margin in a small 

 obtuse angulation, above which the dorsal margin is slightly convex 

 and slopes rapidly from the beaks. 



The inner surface is smooth with inconspicuous muscular scars. 



