96 PALAEONTOLOGY OP THE UPPER MISSOURI. 



pallial line obscure, and remote from the margins. Margin of the right valve in 

 front of the beaks, somewhat thickened, slightly prjojccting, and received into the 

 margin of the other valve, though there are no proper anterior lateral teeth. 



This genus dates back to the Liassic period ; some ten or twelve species having 

 been identified by M. Buvignier and M. Terquem, in rocks of that age, in France. 

 So far as we are' acquainted, it has not yet been recognized above this horizon on 

 the continent, while in England it is only known in the lower Oolite. Ija America 

 it ranges still higher, a single well-marked species having been described by us from 

 Cretaceous beds, at the mouth of Juditli River, on the Upper Missouri. In this 

 country it also made its first appearance in Jurassic rocks. 



This genus is closely allied to the Cretaceous genus Meehia, Gabb (Palaeont. Cali- 

 fornia, I, 18G4, 191), but differs in always having the anterior side closed instead of 

 gaping, as wcU as less angular. There are also some differences in the hmge plate; 

 while the ligament in Meekia is said to be only "subexternal." 



Tancrcdia TVarrenana. 



(Plate III, Kg. 7.) 

 Tancredia Warrenana, Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. May, 1860, 183 ; ib. Oct. 1860, 418. 

 Shell small, trigonal ovate, moderately convex ; anterior half a little narrower and more compressed than the 

 other narrowly rounded at the extremity ; base forming a broad gentle curve ; posterior side subtruncate, angular, 

 or very abruptly rounded below. Dorsum sloping from the beak ; the anterior slope being slightly concave in 

 outline, and the other nearly straight, or a little convex. Beaks rather elevated, but not extending much above 

 the cardinal edge ; posterior umbonal slopes prominent or subangular. (Surface and hinge unknown.) 

 Length, 0.50 inch ; height, 0.33 inch ; breadth, about 0.14 inch. 



We have not yet had an opportimity to see the hinge or the pallial line of this 

 .shell, and consequently only place it provisionally in the genus Tancredia. Its 

 form, however, is such as to leave little room for doubt in regard to its relations to 

 that group. As our specimens are casts, they do not give a -very clear idea of the 

 surface, though it seems to have been only marked by lines of growth. It is only 

 the immediate extremity of the posterior margin that appears to be a little truncated 

 vertically. Our specimens do not show whether the valves were gaping behind or 

 not, but they have the appearance of being closed. 



This species resembles more or less closely several of those figured by Morris and 

 Lycett in their monograph of the fossils of the Great Oolite, but seems to differ 

 specifically from them all, as well as from those figured by Terquem and other 

 continental authors. It agrees most nearly in size and general appearance with 

 T. hrevis, Morris and Lycett (Mol. Gt. Oolite, part 3, fig. 8, pi. xiii, Palaeont. Soc), 

 but is higher in proportion to its length, and more rounded at the extremities. 



Locality and position. — Jurassic beds at southwest base of the Black HiUs. (No. 

 204.) 



Tancredia? aequilaternli^. 



(Plate III, Fig. 8.) 



Tancredia? a-qtiilaleralis, Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. May, 1860, 183 ; ib. Oct. 1860, 418. 



Shell very nearly elliptical, moderately convex ; anterior end rather narrowly rounded ; base forming a broad 

 regular, semi-elliptic curve; posterior end slightly truncate on the upper oblique slope, narrowly rounded below, 

 and apparently not gaping. Beaks depressed, located a little in advance of the middle. Surface of cast retaining 

 traces of concentric strise. (Hinge and interior unknowr..) 



Length, 1 inch; teight, 0.64 inch; breadth, about 0.16 inch. 



