98 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Locality and position. — Yellowstone River, 150 miles above its mouth ; 



in beds containing the fossils of both the Fort Pierre and Fox Hills groups 



(Nos. 4 and 5), where it occurs in immense numbers and in a fine state of 



preservation. 



NUCULIDiE. 



Genus NUCULA, Lamarck. 



Synon.—Nucula, Lam. (1799), Prodr. 87; aud (1809), Phil. Zool. ; and (1819), Hist., VI, 1, 57.— Feruss. 

 (1821), Tab. Syst., xli.— Latr. (1825), Fam. Nat.— Desh. (1832), Eucyc. Meth., Ill, 632.— 

 Bronn. (1835), Letta. (2d ed.),369 aud 929.— Reeve (1841), Couch. Syst., I, 108; aud 

 of many others. 

 Acila, H. and A.Adams (1858), Gen. Keceut Moll., II, 545 (subgenus of ifitcula). — Cheuu (18C2), 

 Man. Conch., II, 179. 



Etym. — Diminutive of nax, a nut. 

 Type. — Area nucleus, Linn. 



Shell transversely-ovate or ovate-subtrigonal, short, and generally trun- 

 cated behind, and longer, with a more or less rounded outline, anteriorly ; 

 beaks near the posterior side and turned backward ; surface smooth, or 

 ornamented by radiating, concentric, or zigzag divaricating stria?- or costse, 

 and covered by an olivaceous epidermis ; hinge angulated under the beaks, 

 and provided at the angle with an oblique, prominent, internal cartilage-pit, 

 on each side of which the small, deeply-interlocking denticles are arranged; 

 ligament sometimes snbexternal ; inner layers highly nacreous. 



H. and A. Adams have proposed to separate this genus into two sub- 

 genera, distinguished as follows : 



1. nucula, Lam. (typical). 



Shell with radiating markings, smooth, or merelv concentrically 

 striate. — (Type as above.) 



2. acila, H. & A. Ad. 



Shell with zigzag divaricating markings. — N. divaricata, Hinds. 



The fact that the truncated and shorter side of the shell in this genus is 

 the posterior, and the beaks are directed backward instead of the reverse, as 

 in most genera of bivalves, gives the animal and the shell the singular appear- 

 ance of occupying reversed positions with relation to each other. What 

 -seems to be the lunule, and is often described as such, also differs in really 

 corresponding to the escutcheon of other types. 



The shells of this genus are readily distinguished from those of Lcada, 

 Yoldia, and Neilo, by their more angular hinge, strongly pearlaceous luster, 



