440 PROCEEDINGS OF THE ACADEMY OF 



CRETACEOUS SPECIES. 



LAMELLIBRANCHIA TA. 



Genus OSTREA Linnaeus. 



OSTKEA GABBANA. 



Shell rather small, thin and longitudinally elliptical or ovate ; cardinal 

 margin narrowly rounded, or subangular at the beaks ; opposite extremity 

 rounded ; anterior and posterior sides forming regular elliptic curves ; sur- 

 face of both valves smooth, excepting obscure marks of growth. Lower valve 

 rounded below, and rather deeply concave within ; beak small, not very pro- 

 minent ; area small, wider than long. Upper valve perfectly flat, nearly 

 smooth, and presenting the same outline as the other, excepting that its beak 

 is rather less prominent ; area short and a little thickened. 



Length from beaks to the opposite extremity, l - 36 inches; diameter from 

 the anterior to the posterior side, 1 inch ; convexity or depth of the lower 

 valve, 0-30 inch. 



This shell presents an unusual regularity of form for an oyster, and is not 

 liable to be confounded with any other species with which we are acquainted. 

 The regular elliptic outline and uniform convexity of the lower valve, with its 

 small beak, ranging exactly in a line with its longer central axis, give it, as 

 seen from below, much the appearance of some smooth species of Terebratula, 

 such as the T. Harlani of Morton. The specimens vary little in form, the 

 only difference being the rather less elongate outline of young individuals. 



Named in honor of Mr. Wm. M. Gabb, of Philadelphia. 



Locality and position. Head Gros-ventres River, from a gray sandstone con- 

 taining Cardium curtum (of this pa^er), a large nearly circular Inoceramus, and 

 an Oyster of undetermined species. The age of this rock is not very clearly 

 determined but is most probably Cretaceous. 



Genus LEDA Schumacher. 

 Leda bisulcata. 



Shell small, transversely subovate ; gibbous in the central region ; anterior 

 side narrowly rounded ; base semi-elliptical, with a slight sinuosity near the 

 posterior extremity ; anal end narrow and subangular in outline ; dorsal bor- 

 der sloping from the beaks at an angle of about 135; beaks rather obtuse, 

 nearly central, or located slightly in advance of the middle ; posterior umbo- 

 nal slopes distinctly angular, the angles extending back to the anal extremity, 

 where they terminate in a small obscure fold ; 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 end of the shell ; lunule not defined ; surface orna- 

 mented by small, rather distinctly elevated, regularly arranged concentric 

 striae. 



Length, 031 inch ; height, 0*16 inch ; convexity, 013 inch. 



At a first glance, this little shell might be mistaken for our Leda, (Yoldia,) 

 scitula (= Nacula scitula Ap. 185G, Proceed. Acad. Nat. Sci., Phila., p. 84) ; 

 but on a closer examination, it will be at once distinguisheu by its angular 

 posterior umbonal slopes and distinctly defined escutcheon, as well as by its 

 stronger concentric striae. It is also a little less gibbous and slightly more 

 depressed. 



Locality and position. Deer Creek, near North branch Platte River, in 

 lower part Fox Hills beds, or the upper beds of Fort Pierre Group of the Ne- 

 braska Cretaceous Section. 



[Dec. 



