INVERTEBRATE PALAEONTOLOGY. 103 



tlif posterior adductor muscle is ovate and bounded by an obtuse ridge, which 

 leaves on the east a shallow groove extending up toward the beaks. The 

 anterior muscular scar is broader than the other, and subtrigonal in form. The 

 radiating costse are obsolete on the lunular and escutcheon-like areas. 



This beautiful Nucula is quite similar to N. pectenata of Sowerby (Min. 

 Conch., II, 207, pi. 192, figs. 6-7 ; and d'Orbigny, in Palaeont. France, III, 

 pi. 303, figs. 8-11), but its costas are decidedly smaller, and its lunule-like 

 depression behind the beaks not near so deep nor so broad, as that of Sowerby's 

 species, being there distinctly cordate in form, while in our shell it is rather 

 narrow-obovate. Its muscular impressions are also much less distinct than 

 in those of A", pectenata, and the crenulations of its free margins finer and 

 more crowded. The points of difference between it and the last have been 

 already noticed in the remarks connected with the description of that species. 



Locality and position. — Moreau River, Long Lake, Deer Creek, on the 

 North Platte, &c; from the Fox Hills group, or No. 5 of the Upper Missouri 

 Cretaceous series. It also occurs on Yellowstone River, Idaho Territory, in 

 the upper part of the Fort Pierre group of the same series. 



LEDIDiE. 



Genus NUCULANA, Link. 



Synon. — Niwulana, Link (1807), Rost. Sauiml., Ill, 15J. — H. and A. Adams (1858), Genera Recent Moll., II, 

 660.— Meek (1864), Smithsonian Check-List. N. Am. Cret. Foss., 8.— Stoliczka (1871), 

 Pah-eont. Indica, III, 320. 

 Leda, Schum. (1817), Ess., 172.— Moll. (1842), in Id-over's Naturh. Tidssk., IV, 91.— Phil. (1845), 



- Arch. f. Nat., XI, 1, 194. 

 Lembulus, Leach (M. S., 1819).— Risso (1826), Hist, IV, 319.— Geinitz (1846), Gnuulr. d. Verst., 420 



(not Sowerby, jr., 1842, = Nucula, Lam.). 

 Dacryomya, Agassiz (1839), Mem. snr les Moul. d. Moll., I, 34 ; and in Germ. Tr. Sowerby's Min. 

 Conch., 500 and 578. 



Etym. — Xucula, a little nut. 

 Type. — N. emarginata, Lam. 



Shell transversely subovate, or more or less attenuated and subrostrate, 

 and closed posteriorly, smooth, or concentrically-striated, and covered with an 

 epidermis; beaks approximate, incurved, and sometimes directed a little back- 

 ward; posterior umbonal slopes often angular; hinge-line slightly angulated; 

 denticles fine, prominent, and usually numerous; cartilage occupying a small 

 internal pit under the beaks ; pallial line with a slight sinus, or sometimes 

 very nearly simple ; internal layer generally a little nacreous. 



The genus Nuculana is nearly related to Yoh/ia, but can generally be 



