INVERTEBRATE PALAEONTOLOGY. 180 



crowded, concentric striae, and sometimes a few obscure parallel furrows near 

 the border. 



Length of an adult shell (oval van), 1 inch ; height, 0.78 inch ; con- 

 vexity, 0.56 inch. 



None of our specimens expose the interior ; but internal casts show that 

 the muscular impressions are faintly marked, and of a rather narrow-ovate 

 form, both before and behind. The pallial line is also seen to be provided 

 with a broad, shallow sinus, having nearly the form of an equilateral triangle 

 Little or nothing being yet known in regard to the teeth of its hinge, it is, 

 like the last, only referred doubtfully to this group. By working carefully 

 about the hinge, however, and grinding down some of the specimens, I have 

 nearly satisfied myself that it has a small anterior lateral tooth as in Merctrix 

 (— Cytherea) and Callista; and as the pallial sinus is distinctly triangular, as 

 in some sections of the latter, I think there is not very much reason for 

 doubting the propriety of placing it, at least provisionally, in the latter genus ; 

 though better specimens may show it to belong to some other group. 



Locality and position. — South Fork of Cheyenne River, near the base of 

 the Black Hills, in the Fort Union group, or formation No. 2 of the Upper 

 Missouri Cretaceous. The specimens first described in the paper cited at the 

 head of this description, were collected from the same formation on the Mis- 

 souri River, five miles below the mouth of James River. 



Genus THETIS, Sowerby. 



Sijnon.— ThtiU, Sowerby (1826), Min. Conch., tab. 013; and German ed., 535 — Defr. (1829), Diet. Sci. Nat., 

 LIV, 274.— Bronn (183S), Leth., 704— Sowerby, jr. (1839), Couch. Man., 106; and 

 (1842), 2d ed., 274.— Gray (1842), Synou. Brit. Mus., 91.— D'Orbiguy, Pal. Fr., Ill, 

 450.— Geinitz (1846), Grundr. d. Verst., 419.— Chenu (1862), Man. de Conch., II, 90 

 (not Tethijs, Linn., 1740, 1758, and 1767, sometimes written Thetya and Thetis ; nor 

 Tlielh, Adams, 1845). 

 Thetironia, Stoliczka (1870), Palasont. Iudica, III, 158. 



Etijm. — Qctlq, a Nereid, the wife of Peleus. 

 Exam}). — T. major, Sowerby. 



Shell equivalve, subequilateral, subtrigonal, or more or less nearly 

 orbicular, ventricose, but thin in substance, closed all around ; surface con- 

 centrically striate, and often showing minute punctures under a magnifier; 

 ligament external.; hinge with three (or four?) cardinal teeth in each valve, 

 the anterior being large, conical, and slightly curved, and the posterior small; 

 lateral teeth none; muscular impressions broad-oval; pallial line distinctly 

 marked, and provided with a very profound, sharply angular sinus, directed 

 upward, and often extending almost as high as the back part of the beaks. 



