INVERTEBRATE PALAEONTOLOGY. 105 



dorsal margin, to the upper part of the subangular extremity, where they 

 terminate in a small fold bordered below by an obscure, oblique sulcus; 

 escutcheon lanceolate, concave along the middle, and strongly defined by the 

 umbonal angles, just within which there is, on. each side, a narrow, well- 

 defined sulcus, extending from the beaks to the posterior extremity of the 

 shell ; lunule not defined ; surface ornamented by small, rather distinctly- 

 elevated, regularly-arranged, concentric strise. 



Length, 0.31 inch; height, 0.16 inch; convexity, 0.13 inch. 



At a first glance, this little shell might be readily mistaken for our Yoldia 

 scitula, with which it closely agrees in size and form. On comparison, how- 

 ever, it will be at once distinguished by its angular posterior dorsal umbonal 

 slopes, and distinctly-defined escutcheon, with a sulcus just within each of its 

 lateral margins, as well as by its stronger concentric stria?. It is also a little 

 less gibbous and slightly more depressed. I know nothing of its hinge or 

 internal characters, but there is something in the expression of its posterior 

 dorsal region, with its angular umbonal slopes, that indicates relations rather 

 with Nuculana than Yoldia. 



Locality and position. — Deer Creek, near North Branch of Platte River; 

 in the lower part of the Fox Hills beds, or the upper part of the Fort Pierre 

 group of the Cretaceous. 



Nuculana subnasuta, H. &M. 



Plate 15, rig. 0. 



Nucula subnasuta, Hall and Meek (1854), Mem. Am. Acad. Arts ami Sci. Boston, V, new series, 384, pi. 



1, fig. 10. 

 Leda (Yoldia) subnasuta, Meek and Haydeu (Oct., i860), Proceed. Acad. Nat. Sci. Philad., XII, 429. 

 Nuculana subnasuta, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 8. 



Shell transversely subelliptical, nearly or quite equilateral, moderately 

 gibbous in the umbonal region ; extremities rather narrowly-rounded or sub- 

 angular, the anterior margin being a little more obtuse than the other; pos- 

 terior side having a shallow, undefined groove or depression extending 

 obliquely from the beaks to the postero-basal margin, where it produces a 

 very slight emargination ; escutcheon margined by a faint, shallow groove, 

 which extends from near the beaks to the upper part of the posterior extrem- 

 ity; beaks small, incurved, and approximate. Surface marked by regular 

 concentric striae, which become nearly obsolete on the extremities ; crenula- 

 tions of the hinge very fine. 

 14 H 



