232 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



widened horizontally, and extending out at right angles from beneath the 

 beak, with a slight upward curve, as it were, from close up under the hinge ; 

 its free end being transversely truncated. Partly over and slightly in advance 

 of the anterior margin _ of this tooth, there is a second well-defined, but 

 smaller, pointed cardinal, directed in the same way as the other. Partly 

 between these, or mainly under the smaller tooth, there is a deep pit, for the 

 reception of the larger cardinal of the other valve ; while behind this pit, 

 over the posterior part of the larger tooth, there is a shallower pit, directed 

 nearly horizontally backward, just beneath the cardinal edge, for the recep- 

 tion of the smaller rudimentary tooth of the other valve; the two pits 

 mentioned being separated by a very minute vertical ridge, extending down 

 from directly under the very point of the beak. In the left valve (Cymdla), 

 there is just in front of, and below the apex of the beak, a well-defined, 

 subtrigonal. rather pointed, conical, anterior cardinal tooth, flattened on top 

 and a little curved upward, extending out at right angles to the hinge — this 

 being the tooth that occupies the larger pit mentioned, partly between the 

 two teeth of the other valve — that is, passing mainly under the smaller and 

 partly above the anterior side of the larger. Immediately over this principal 

 cardinal of the left valve, there is a small pit for I he reception of the ante- 

 rior or smaller tooth of the other valve; and just behind this, there is some 

 appearance of a minute vertical ridge, similar to that seen in the other valve ; 

 while a little farther back, there is a slight, nearly horizontal projection, or 

 second rudimentary posterior cardinal tooth (that might be easily overlooked 

 without a careful examination), which, when the valves are united, fits into 

 and exactly fills the little horizontal pit mentioned, over the posterior and 

 larger tooth of the other valve. Directly under this little horizontal tooth 

 of the left valve, the under-side of the cardinal margin shows a rather wide 

 excavation for the reception of the larger flattened posterior tooth of the 

 other valve, which thus passes under the margin of the left valve. None of 

 the specimens examined show any traces of lateral teeth ; nor have I yet 

 seen any in a condition to show the muscular and pallial impressions.* 



Notwithstanding the fact that my friend Mr. Conrad has adopted both 

 Ijiopistha and Gymella as distinct genera, and I am, as might be expected, 

 naturally desirous that they should lie both sustained as such, if it can be 



" Mr. Conrad describes CyTnella us being pearly. None of those I have seen, however, show this 

 character clearly. 



