94 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



NUCULID^l. 

 Genus NUCULA Lamarck. 



NUCULA OOLORADOENSIS n. Sp. 



PI. xxi, Fig. 9. 



Shell small, transversely ovate subtrigonal, moderately convex; dor- 

 sal margin in front of the beak slightly convex, declining to the sub- 

 angular anterior end ; posterior end very short, truncate almost at right 

 angles to the dorsum; basal margin forming a regular cfarve, finely 

 cren ate within; beak rather prominent, incurved, very near the pos- 

 terior end of the valve. Surface marked by faint, closely arranged 

 radiating lines and by large irregular concentric ridges and lines of 

 growth. 



Length, 12 mm ; height, 10 mm ; convexity of single valve, 3 mm . 



Only a single well preserved valve of this little shell was found, and, 

 not being able to identify it with any of the described Cretaceous spe- 

 cies from the Rocky mountain region, I have thought it best to give it 

 a provisional name. The fact that the most nearly related forms come 

 from a much higher horizon renders it more probable that this is a dis- 

 tinct species. In form it is similar to both Nucula planimarginata M. 

 & H. and Nucula cancellata M. & H., but the posterior end of our spe- 

 cies is proportionally much shorter and there are other slight differ- 

 ences in outline. In surface ornamentation it is more nearly like JY. 

 planimarginata, but its crenate inner margin would prevent its identi- 

 fication with that species. 



Locality and position. — On Williams creek, Huerfano park, Colorado, 

 in concretions near the base of the Fort Benton shales, associated with 

 Crassatella excavata. 



Genus YOLDIA Moller. 



YOLDIA SUBELLIPTICA U. Sp. 



PL xxi, Figs. 6 and 7. 



Shell small, thin, transversely elongate subelliptical in outline, mod- 

 erately convex in the umbonal region, depressed towards the extremi- 

 ties; dorsal margin declining slightly and equally in both directions 

 from the beaks; both ends of the shell subangular above and broadly 

 rounded below; basal margin almost straight; beaks rather prominent, 

 tumid, situated a little behind the middle. 



Surface marked by minute concentric lines. Hinge and interior fea- 

 tures unknown. 



Length, 7 mm ; height, 4 mm ; greatest convexity of both valves united, 



Ctrl in 

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Not having seen the hinge of this species I am not certain that it 

 belongs to the genus to which it is provisionally referred. 



Locality and position. — In the Pugnellus sandstone, Poison canyon, 

 Huerfano park, Colorado. Only one specimen has been found. 



