INVERTEBRATE PALEONTOLOGY. 



59 



it .seems to have varied somewhat in this respect. It is also usually less 

 gibbous, has smaller beaks, and a more regular oval outline tlian most of its 

 associates. Our figure represents a young shell. 



Locality and position. — This species is unknown to us from any other 

 locality than the Great Bend of the Missouri, below Fort Pierre; where it 

 occurs in the base of the Fort Pierre group, or No. 4 of the Cretaceous. 



1 1 1 1 n ■ « ■ r : i 1 1 1 1 1 <■ tenuirostris, M. & H. 



Itioccramim tenuirostris, Meek and Hayden (18(a), Proceed. Acad. Nat. Sci. Philud., XIV, 27. 



Left valve very gibbous, subquad- 

 rate in outline ; anterior margin very 

 short, or vertically truncated, with a 

 slightly convex outline, immediately in 

 front of the beak, and rounded into the 

 base below ; ventral margin nearly semi- 

 elliptical; posterior side rounded, or some- 

 times subtruncated, with a slightly convex 

 outline above, and a little more prominent 

 and rounding into the base below ; hinge 

 of moderate length, with cartilage-furrows inoceramus tenuirostris. 



small, there being about five of them in side-view of a cast of left valve, 



a space of 0.20 inch ; beak very gibbons, prominent, narrowed, strongly 

 incurved, and directed a little forward, its point being immediately over the 

 anterior margin. Surface of internal cast smooth over the gibbous umbonal 

 region, but showing traces of small, concentric undulations below the middle. 

 (Right valve unknown.) 



Length, 2.10 inches; height from base to hinge, 1.82 inches; height to 

 top of umbo, 2.13 inches; convexity of left valve, 0.90 inch. 



This shell has a more prominent, attenuated, and strongly-incurved left 

 beak than any other species of the Catillus group with which I am 

 acquainted; being in this respect more like many species of typical Inoce- 

 ramus, while its general form - places it with the former section. No speci- 

 mens (if its right valve are yet known, but it is probably much less convex 

 than the left; I know of no other species with which it is liable to be con- 

 founded. 



Locality and position. — Chippewa Point, Montana; Fort Benton group. 



