154 University Geological Survey of Kansas. 



Yoldia knoxensis? Plate XX, fig. 4. 



Leda polita McChesney, New Pal. Foss., p. 53, (1860). 

 Leda knoxensis McChesney, 111. New Spec. Foss., pi. u, ff. 9a-c, (1865). 

 Yoldia knoxensi* McChesney, Trans. Chic. Acad. Sci., i, p. 39, pi. n, ff. 

 9a-c, (1868). 



Shell rather large for this genus, ovate in outline. The hinge 

 line in front of the beaks bends somewhat downward ; the en- 

 tire anterior and ventral border from near the beaks around to 

 the antero-dorsal border forming a regular, perfectly ovate 

 curve, the broad end of which is placed foremost ; postero- 

 dorsal border rounded more sharply but not angulated, turning 

 considerably forward to meet the hinge, in an almost straight 

 line. Hinge, back of the beak, nearly straight ; teeth numerous 

 and very fine ; valves moderately convex ; umbones not promi- 

 nent ; beaks closely approximated and incurved. Greatest 

 convexity in front of the middle of the shell ; the greatest 

 vertical diameter near or a little in front of the center ; beaks 

 placed about one-third the distance from the front end of the 

 hinge. There is a faint depression extending from back of 

 the umbo, near the hinge line, obliquely backward over the 

 umbonal ridge but soon vanishing. This is so faint as to be 

 hardly noticeable, yet it is present in every specimen before me. 

 Surface ornamented by very fine, regular, concentric lines of 

 growth. Length, 28 mm.; height, 16 mm.; convexity (both 

 valves), 8 mm. Specimen a little larger than the average. 



Range and distribution : Upper Coal Measures ; Leavenworth, 

 Lansing coal-shaft. 



This shell agrees very well with McChesney 's description of 

 Y. knoxensis, Proc. Chic. Acad. Nat. Sci., i, pi. n, f. 9, but dif- 

 fers from the figure of the species in being about as wide just 

 back of the beaks, near the center of the shell, as at any other 

 place, in being less attenuate behind, less abruptly rounded on 

 the posterior extremity, and meeting the hinge line at a very 

 much greater angle than is shown by his figure, which shows 

 the posterior extremity of the shell to be subtruncate. 



