38 PALEONTOLOGY OF THE UPPER MISSOURI. 



very gradually behind, and ronnding up more abruptly in front ; dorsal margin nearly parallel to the base, or 

 declining a little posteriorly. Surface of internal casts marked by rather small, moderately distinct concentric 

 undulations, which are quite regular over the umboues, but become more irregular and obscure towards the 

 margins of the valves. 



The hinge is long, and nearly straight. In casts there is a rather narrow depression extending along its entire 

 length, bounded on either side by an obtuse ridge, ranging parallel to the hinge line. Immediately outside of each 

 of these ridges there is a shallow rounded sulcus, which is widest near the posterior muscular scar, and becomes nar- 

 rower, and less distinct towards the beaks. The anterior muscular impression is well defined, transversely lunate iu 

 form, and connected with the pedal scar above in such a manner that the two together present much the appearance 

 of a capital letter G, lying on its back. Tile posterior muscular scar is large, broad rhombic-subovate in form, and 

 placed near the hinge, about one-third the entire length of the shell from the posterior extremity. The pallial line 

 is faintly marked, and apparently provided with a deep acutely angular sinus. 



Length, 5.10 inches ; height, 2.25 inches ; greatest thickness or convexity near the anterior end, 1.70 inch. 



This species is very closely allied to Sanfjuinolites clava, of ^IcCoy (cited at the 

 head of this description), and may possibly prove to be identical, when a direct 

 comparison of specimens can be made. Those we have yet seen, however, of the 

 Kansas fossU, differ from Prof. McCoy's figures in being straighter on the dorsal 

 margin, and more produced, as well as more narrowly rounded in the antero-ventral 

 region. Their concentric undulations are also more obscure, and the lunule-like 

 depression in front of the beaks less distinctly defined, in our shell. 



Local Uij and position. — Leavenworth City, Kansas, from a bed holding a posi- 

 tion a few feet above low-water mark of the Missouri. Coal Measures. (Type 

 1020.) 



Genus SEDGWICKIA, McCoy. 



Si/non. — Sedgwichia, McCoT, Synopsis Garb. Foss. Ireland, 1844, 61. 



Lepludomus, McCoy, British Pal. Foss. 1852, 277 (not 1844, Carb. Foss. Ireland, p. 66). 



Sayiguiiwlitcs (part), McCoT, Brit. Pal. Foss. 1852, 276 (not Carb. Foss. Ireland, 1844, p. 47). 

 Etym. — Dedicated to Kev. Adam Sedgwick, M. A., F. R. S. 

 Type. — Sedgwickia atlenuatn, McCoy. 



Shell depressed oblong, or subo'^al, nearly or quite equivalve, inequilateral, very 

 thin ; anterior side not quite closed, often a little gibbous ; posterior side longer, 

 more compressed, and more widely gaping. Beaks prominent, rather tumid, in- 

 curved ; posterior umbonal slopes prominently rounded, or sometimes forming an 

 oblique ridge, generally separated from the compressed postero-dorsal region by a 

 shallow, linear sidcus, which is also sometimes marked on internal casts. Flanks 

 compressed, or somewhat concave in the antero-ventral region, or a little behind it. 

 Lunular impression distinct. Surface finely granulose, and ornamented with more 

 or less regular concentric ridges and stria?, the ridges being usually obsolete on 

 the posterior and compressed postero-dorsal portions of the valves. Hinge edentu- 

 lous ; cardinal margin inflected so as to form a narrow false area behind the beaks. 

 (Muscular and pallial impressions very obscure and not well kno^vn ; animal un- 

 known.) 



This group, as here defined, agrees in all essential characters with Leptodomus, 

 as characterized in 1852 by Prof. McCoy, in his beautiful work on the British 

 Palaezoic Fossils. We think, however, that this name cannot be retained, in 



