94 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



and posterior muscular impressions are bounded by slightly prominent linear 

 ridges extending up toward the beaks. 



This little shell seems to agree exactly in all respects with Mr. Conrad's 

 group Breviarca, excepting that there is a single cartilage-furrow divaricating 

 from near the middle, so as to inclose a triangular space between the beaks, 

 in the hinge-area of each valve; but it also has the minute cross-striae seen 

 on the typical species of Breviarca, though they are confined to the central 

 triangular space of each area, instead of occupying the whole. I cannot 

 believe, however, that this slight difference is of even subgeneric importance. 



Specifically, this shell seems to be most nearly allied to T. (Breviarca) 

 Saffbrdi, Gabb (sp.), but it is proportionally higher and shorter as well as 

 more gibbous, while nothing is said in Mr. Gabb's description of that species 

 respecting any such peculiarities of the hinge-area as those seen in our shell. 



Locality and position. — Mouth of Milk River, on the Missouri, and near 

 the forks of Cheyenne River; in the upper beds of the Fort Pierre group, or 

 No. 4 of the Nebraska Cretaceous series. 



Genus AXINJEA, Poli. 



Synon.—Axinaia, Poli (1791), Test. Utr. Sic, I, 32.— H. and A. Adams (1858), Gen. Eecent Moll., II, 541.— 



Meek (1864), Smithsonian Check-List N. A. Cret. Fossils, 8.— Gabb (1864), Paheont. 



California, I, 197 ; and ib., II (1869), 196. 

 Axiiiceoderma, Poli (1795), Test. Utri. Sic, II, 254.— Oken (1815), Zool. 

 Glycimeria, Humphrey (1797), Mus. Col., 50 (not of Lam. and others). 

 Tuccta, Bolteu (1798), Mus. Bolt., 102, 2d ed. 

 PectuncuUs, Lam. (1799),Prodr.,89; and (1801), Syst., 115.— Koissy (1805), Moll., VI, 404— Feruss. 



(1821), Tabl. Syst., xli.— Rang (1829), Man., 28*.— Bronu (1838), Leth., 935.— Sow., jr. 



(1S42), Conch. Man., 2d ed., 218.— Hanley (184C), 111. Cat., 163.-D'Orbigny (1844), 



Pal.-eont. Fr., Ill, 186 ; and of many others. 



Etym. — dfu>j;, an ax. 



Examp. — Area glycimeria, Linn. 



Shell orbicular, equivalve, nearly or quite equilateral; margins closed, 

 smooth or crenate within; surface ornamented with radiating costse, concen- 

 trically striate, or nearly smooth, covered by a fimbriated epidermis; beaks 

 not oblique; ligament-area well developed and divaricately furrowed ; hinge 

 arcuate and provided with radiately-arranged denticles; muscular impressions 

 and pallial line well defined, the former generally with somewhat projecting 

 margins, but without a distinct plate. 



H. and A. Adams propose to separate as a subgenus, under the name 

 Pcctunculus, the species of Axitma that are ornamented by prominent radiat- 

 ing cosfa3. Among the fossil species, however, there are so many gradations 



