oi{ETA(:eo(is fossils. 157 



or quite equilateral; anterior margin narrowly rounded; base forming- a 

 semi-elliptic curve; jwsterior end narrowly rounded or subangular at the 

 termination of a slight, oblique flexure of the valves; dorsal mai-gin sloping 

 from the beaks to the extremities, with a moderately convex outline; beaks 

 almost central, rather small, and projecting little above the cardinal margin, 

 incurved with scarcely any visible obliquity; hinge and interior unknown, 

 Surface ornamented by fine, perfectly regular, concentric, thread-like Imes, 

 gradually becoming smaller and more crowded toward the undjones, on 

 which they are nearly or quite obsolete. 



Length, 0.90 inch; height, 0.60 inch; convexity, about 0.20 inch. 



The fact that this is one of the characteristic shells of the formation in 

 which it occurs seems to render it desirable that some notice should be 

 taken of it, although we have not the means of arriving at satisfactory con- 

 clusions in regard to its affinities. At the same time that I place it pro- 

 visionally in the above genus, I really have very little faith in its belonging 

 propei-ly to that group. It seems to be a very thin shell, as the spec- 

 imens (which in some instances appear to be internal casts) show the 

 exceedingly regular thread-like concentric lines quite distinctly. In some 

 of these casts there are appearances of the impression of a thin lamina, or 

 very slender ridge, nearly parallel to the hinge-margin both behind and in 

 front of the beaks. These may have been left by elongated lateral teeth; 

 though they appear not to be exactly marginal, but a little removed from it, 

 as if they had been made by a thin projecting lamina, somewhat similar 

 to the cartilage-support in the genus Edmondia. I do not think the shell at 

 all related to that group, however. 



After numerous comparisons, I have been unable to identify this shell 

 with any described species. 



Locality and posit'ton. — In whitish and buft'-colored sandstone, at Chalk 

 Creek, above Coalville, Utah; also in a similar matrix from East Canon 

 Creek, Wasatch Range; Cretaceous. 



TKLLINA MODESTA, .MtH'k. 

 Plate 15, figs. 4 and 5. 



Shell rather small, transversely elongate-subelliptic, being twice as long 

 as high, rather distinctly compressed; anterior margin narrowly rounded; 



