222 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



beaks being united by short horizontal bars. Perfect specimens would 

 doubtless show other corresponding differences. 



It is probably more nearly allied to a species described by me from 

 Vancouver's Island (Trans. Albany Inst., IV, 41), under the name of Phola- 

 domya (Goniomya) borealis, but it is proportionally narrower, and has less 

 elevated beaks. 



Locality and position. — Moreau River, Dakota Territory ; No. 5, or Fox 

 Hills group of Nebraska Cretaceous series. 



ANATINID^E. 



Genus THRACIA, Leach. 



Synon.—Thraeia, Leach (1819), MS. ; Blainville (1824), Diet. Sci. Nat., XXX, 347 ; and (1825) in Maiacol., 

 504.— Rang (1829), Man., 324— Desbayes (1830), Encyc. M<5th., Ill, tab. 1832 ; ib., 

 1038 : and (1835) in Lam., 2e eU, VI, 82.— Menke (1830), Synon., 2e e"d., 119, &c. (not 

 Thracia, Westwood, 1840 ; a genus of insects). 



Odoncinetus, Pacosta (1829), Cat. Syst.,32. 



Odontocineta, Agassiz (1840), coir, in Index Univ., 255. 



Shell transversely ovate or oblong, thin, inequivalve, compressed or 

 moderately convex ; posterior side somewhat narrowed, usually truncated 

 and more or less gaping ; surface concentrically striated, sometimes minutely 

 scabrous ; beaks entire ; cardinal margin behind the beaks a little thickened, 

 inflected, and somewhat projecting inward, so as to form in each valve a kind 

 of cartilage-process ; cartilage provided with a free crescentic ossicle ; liga- 

 ment partly internal ; muscular impressions small; pallial sinus moderately 

 deep. 



This genus seems to be nearly related to Corymya, Agassiz ; and it is 

 thought by some that the latter is not generically distinct. According to 

 Stoliczka, however, Corymya is provided with two more or less diverging, 

 long, internal ribs, running from the beaks posteriorly and downward. The 

 hinge-margin behind the beaks, although a little thickened, seems also to 

 differ in not being so inflected and produced as in Thracia. Whether or not 

 the pallial line in Corymya agrees with that of Thracia is yet undetermined ; 

 none of the numerous species of the former yet known showing any traces 

 of it. 



In regard to the period at which the genus Thracia made its first appear- 

 ance, different opinions are entertained, owing to the difficulty of distinguish- 

 ing the fossil species from those of Corymya. The opinion most generally 



