90 University Geological Survey of Kansas. 



by numerous slender, exceedingly regular, closely arranged 

 concentric lines, extending parallel with each other and the 

 front and lateral margins. Ventral valve unknown. Length 

 of medium-sized specimen, 1.3 inches; breadth, 2.95 inches." 



Range and distribution : Upper Coal Measures ; Kansas City, 

 Lawrence. 



Two small specimens very similar in form and markings to 

 the one above described were collected by the writer at Topeka. 

 They were only about an inch in width and only the cast of the 

 shells remain. They are probably specifically different, but 

 too poorly preserved to admit of description. There is a very 

 large form in the University collection which is quite convex 

 and seems to possess an extra number of "platforms" from 

 the one figured by Hall and Clarke, and may belong to a dif- 

 ferent species. It is figured in plate XII, figure lb. 



Rhipidomella pecosi. 



Orthis pecosi Marcou, Geol. N. Amer., p. 48, pi. vi, f. 14, (1858); etc. 

 Orthis carbonaria Swallow, Trans. St. L. Acad. Sci., i, p. 215, (1858); 

 Meek, Fin. Rep. U. S. Geol. Surv. Neb., p. 131, pi. i, f. 8, (1872); etc. 



Meek's description : " Shell small, suborbicular, slightly 

 wider than long, moderately convex in adult specimens ; lateral 

 margins rounded, or, in some examples, faintly straightened 

 posteriorly ; front more broadly rounded, but usually very 

 slightly sinuous in the middle ; valves nearly equally convex ; 

 hinge line very short, or only equaling about half the breadth 

 of the valves. Ventral valve usually most convex in the urn- 

 bonal region, sometimes a little flattened anteriorly, so as to 

 give the shell slightly the form unusually called ' resupinate,' 

 though in gibbous specimens this character is nearly obsolete ; 

 beak moderately prominent, rather pointed and arched ; area 

 small, well defined, and arching with the beak ; foramen nar- 

 row. Dorsal valve usually most convex between the middle 

 and the beak, which is small, and nearly as prominent and 

 arched as that of the other valve, generally with a shallow 

 sinus extending from the middle .to the front ; area well de- 

 veloped but smaller than in the other valve, arched, and divided 



