254 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



It is probably more nearly allied to Leptosolen Mbreanus, d'Orbigny (sp.), 

 already cited, but differs from d'Orbigny's figure in being decidedly straighter 

 on the dorsal margin, and wider as well as more distinctly truncated poste- 

 riorly. 



Locality and position. — Twelve miles southwest of Salina, Kansas ; 

 Dakota group of the Cretaceous series of the Upper Missouri. Discovered 

 by Professor Mudge. 



PHOLADID^E. 



Genus TUENUS, Gabb. 



Synon— Turnus, Gabb (1864),Palseont. California, I, 145.— Stoliczka (1870), Palasont. Indies, III, 19. 



Gonioohasma, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 34 (subgenus). 



Xylophagella, Meek (1864), ib. (subgenus). 

 Etym. — Turnus, a prince, of the Butuli. 

 Type. — Turnus plena*. Gabb. 



Shell varying from transversely-oval to globose; valves closed or some- 

 what gaping behind and open in front; anterior hiatus large, formed by the 

 oblique truncation of the anterior ventral margins of the valves, or a deep 

 rectangular notch of the same; dorsal margins of the valves in front of the 

 beaks ascending and recurved ; beaks more or less incurved ; surface concen- 

 trically striate or costate, with a well-defined oblicpie or vertical linear 

 umbonal sulcus ; interior strengthened by two ridges in each valve, the ante- 

 rior one of which is narrow, smooth, or strongly crenate, and corresponds 

 exactly to the position and direction of the external umbonal furrow, while 

 the posterior one is usually broader, smooth, and directed obliquely backward 

 and downward from the beaks to, or toward, the posterior basal margin. 

 (Accessory valves, if there were any, unknown.) 



The foregoing diagnosis is drawn up so as to include, along with the 

 typical form of the genus, two other types, that I am now inclined to believe 

 cannot properly be separated more than subgcnerically from each other, or 

 from Mr. Gabb's genus. The three sections or subgenera may be severally 

 characterized as follows : 



1. turnus, Gabb (typical). 



Shell transversely oval ; anterior hiatus formed by the oblique 

 truncation and slightly sinuous outline of the anterior ventral margins 

 of the valves ; umbonal sulcus and both of the internal ridges very 

 oblique, narrow, smooth, and extending to the free margins. — (Type 

 as already stated.) 



