MOLLUSCA OF THE ' CHALLENGER ' EXPEDITION. 



517 



13. Dentalium didymum, W. (hi^vfxos, as f^t-o-sided. Also from 

 St. Thomas, also as doubtful.) 



St. 24. St. Thomas, N. of Culebra Island, Danish W. Indies. 

 390 fms. Mud. 



Shell. — Extremely attenuated, very slightly curved, a little flat- 

 tened laterally, and that chiefly toward the convex curve, so 

 that the form is slightly trigonal, porcellanous, pure white, 

 brilliant. Scitlpture. Very fine, irregular scratches run round 

 the shell, the surface of which is not perfectly uniform ; there 

 are very faint indications of longitudinal texture, and there 

 is in the substance of the shell a certain transverse flocculence. 

 Towards the mouth the shell is extremely thin as usual ; but 

 towards the apex it becomes thick from the smallness of the 

 bore, which lies not in the centre but nearer the convex curve 

 of the shell. L. 1-08. B. 0-06, at apex 04<. 

 This measure is taken from the largest of six fragments, none 



of which preserve the apex of the shell. 



14. DsNTALiril TOKOHAMENSE, W. 



St. 233. May 17-26, 1875. Yokohama, Japan. 8-14 fms. 

 Mud. 



Shell. — Much curved when young, becoming nearly straight with 

 later growth, little conical, rather strong, opaque, yellowish 

 white, quite dull, but not chalky. Sculpture. Irregular, 

 slightly elliptical, lines of growth, a little puckered, generally 

 slight, but sometimes sharp and even ; towards the mouth 

 faintly imbricated ; occasionally marked by a deep furrow- 

 like constriction of the shell. The longitudinal rihs are 8 

 to 9 in number, equal, rounded, rather strong, but not very 

 prominent. These are parted by furrow^s, round and open, 

 very shallow, and of very unequal breadth. In these furrows, 

 one, two, or even three thread-like riblets appear, and in the 

 whole texture the lens shows a tendency to a longitudinal rod- 

 like structure. At the apex the shell is squarely truncate, 

 and in the young shell there is, on the convex slope, a slight 

 ragged fissure. L. 12. B. at mouth 0*15, at apex 0*003. 

 The ribs here are much less sharp than they are in D. dentalis, 

 Jj., and there is no trace of the exquisite longitudinal fretted stria) 

 which cover the furrows in that species. The sharp intercostal 

 strise of D. octogonum are quite absent here, and in that species, 

 which is much more bent, the ribs are much wider apart and 

 more equally parted. 



