198 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



while the substance of its shell is much thinner than that of T. equilateralis. 

 It approaches more nearly T. striata, Sowerby (Min. Conch., V, 79, pi. 45(5, 

 fig. 1), but wants the radiating striae seen near the posterior margin of that 

 species. 



So far as I have been able to determine from the examination of casts, 

 this species seems to have no well-defined lateral teeth, and from this fact 

 and its resemblance in form to some species of that section, I place it doubt- 

 fully in the group Peronaa. 



Locality and position. — Abundant at Long Lake, and on the Missouri 

 above Fort Pierre ; in the Fox Hills beds, or No. 5of the Upper Mo.Cretaceous 

 series : also on Moreau River, and the South Branch of Cheyenne River, in 

 the same geological position. 



Genus LINEARIA, Conrad. 



Synon. — Arcopayia (.sp.), d'Orbigny and others; not Leach. 



Linearia, Conrad (1860), Jour. Acad. Nat. Sci. Philad., IV, 279 ; and (1871) Am. Jour. Conch., VI, 



73 ; also (L873), Appendix to Kerr's Geol. Rep. S. Carolina, 0. 

 Liolhyris, Conrad (1873), ib., 9 (as a subgenus of Line aria) 



Etym. — Linearia, linear ; in allusion to the lined surface. 

 Type. — Linearia metastriata, Conrad. 



Shell equivalve, subequi lateral, transversely oblong, or more or less oval, 

 with extremities rounded or posterior margin -a little truncated ; valves rather 

 compressed, or moderately convex, and very slightly or not at all flexuous 

 behind; surface generally partly or entirely marked by radiating striae, and 

 more or less defined concentric lines ; hinge with two slightly diverging 

 cardinal teeth in each valve, directed very obliquely forward and downward ; 

 lateral teeth long, sometimes stronger in the left than the right valve, or the 

 reverse ; pallial sinus usually rather narrow, rounded at the end, directed 

 obliquely forward and upward, and reaching to, or a little beyond, the middle 

 of the valves. 



Mr. Conrad proposes to separate the species without radiating striae as 

 a subgenus Liothyris ; but the transition from species with distinct radiating 

 lines to those with merely concentric striae seem to be so very gradual that 

 it will perhaps hardly be possible to separate them into two well-defined 

 subgenera. 



According to Dr. Stoliczka, of the Indian survey, who has had an oppor- 

 tunity to study the hinge of an Indian Cretaceous species referred by him to 



