CRETACEOUS FOSSILS. 153 



rather convex, and but very slightly oblique; beaks nearly central, rather 

 j)rominent, distinctly incurved almost at right angles to the hinge ; posterior 

 margin truncated with a slight forward obliquity, so as to connect with the 

 dorsal margin at an obtuse angle; anterior margin rounding regularly into 

 the base, which describes a slightly oblique semi-ovate curve, being more 

 prominent behind, where it rounds up very abruptly to the posterior mar- 

 gin, so as to give a subangular outline to the posterior basal extremity ; 

 umbonal slopes rather prominent, but not angular; hinge-margin shorter 

 than the length of the valves. Surface ornamented by small, regular, sim- 

 ple radiating costse, and moderately distinct lines of growth. 



I have seen specimens nearly twice the linear dimensions of that figured 

 on plate 15; and, as these have the posterior umbonal slopes rounded instead 

 of angular, there seems to be very little reason for doubting that it is a dis- 

 tinct species from the last. As already stated, I at first thought it the young 

 of C. curkmi; but the specimens I have since had an opportunity to examine 

 have led to a different conclusion. 



Locality and position. — Chalk Hill, near Coalville, Utah, where it occurs 

 in a whitish sandstone of the coal-bearing Cretaceous series of that region. 

 I also collected specimens of it between Coalville and Weber Canon, at 

 apparently higher horizons than the Chalk Hill beds. 



MACTRID^. 



Genus MACTRA, Linnaeus. 

 Mactka? Emmonsi, Meek. 



Plate 15, fig. 8. 



Shell small, oval-subtrigonal, rather compressed, longer than higli, 

 nearly or quite equilateral, or with anterior side slightly longer than the 

 other; basal margin forming a semi-elliptic curve; anterior margin narrowly 

 rounded below the middle; posterior margin somewhat broader; most promi- 

 nent and abruptly rounded or obtusely subangular below, and very faintly 

 subtruncated obHquely above ; dorsal margin sloping before and behind the 

 beaks, the anterior slope being greater, with a concave outline; beaks nearly 

 central, or sometimes placed a little behind the middle, rather depressed, and 

 incurved with very slight obliquity; posterior umbonal slope very obscurely 



