MOLLIJSCA OF THE * CHALLENGER ' EXPEDITION, 527 



in not being compressed. It is also smaller. It is extremely 

 like G. Jeffrey si, Monter., but is a little more bent, especially in 

 front, is larger, and seems a thinner shell. 



7. Cadtjltjs ctjetus, W. 



St. 24. March 25, 1873. Culebra Island, St. Thomas, D. "W. 



Indies. 390 fms. Mud. 30 specimens. 



Shell. — Short, broad, narrowed at both ends, scarcely bent, and 

 that almost wholly near the mouth ; swollen in the middle, so 

 as to bulge a little on the concave curve. Though the mouth 

 is larger than the apex, the whole shell is a little more pinched 

 in in front than behind, and is very slightly laterally com- 

 pressed (in the proportion of about 14 to 15). It is thin, po- 

 lished, translucent white, with one, sometimes two, opaque rings 

 near the apex. Sculpture. Only under a high power of the 

 microscope can some very close transverse striae be seen in the 

 texture of the shell. Moutli rather large, very slightly oblique ; 

 edge thin and generally much chipped. Apical opening small, 

 straight, chipped. The opaque rings result from thickening, 

 caused by a thin projection which narrows the opening. L. O'l. 

 B. at mouth 0-019, thickest 0-03, apex 0-012. 



Than var. congruens,'W ., this is not only very much smaller, but 

 here the mouth is oblique, and the shell straighter behind and 

 more bent in front, where, too, it is more pinched in. It is nearly 

 of the same length as C. oiesus, W., but is very much narrower. 



7a. Cadultts citetus, var. coNGinjEisrs, JV. 

 St. 24. March 25, 1873. Culebra Island. St. Thomas, Danish 

 "W. Indies. 390 fms. Mud. 2 specimens. 



This differs from C. curtus, "W., in being one third larger ; the 

 mouth is perhaps less oblique, but being in both specimens much 

 chipped, this may be accidental. The most remarkable feature of 

 difference is that it is perfectly round, <md not, like the other, 

 laterally compressed. I attribute this difference to age. At all 

 events, in the absence of a larger series of specimens, I believe it 

 safer to include both under one species. 



8. Cadultjs oBESirs, W. 



St. 24. March 25, 1873. Culebra Island, St. Thomas, D. W. 

 Indies. 390 fms. Mud. 10 specimens. 



