Bkkdk.] Carboniferous Invertebrates. 97 



rather distinctly arched or incurved; area arched, usually of 

 moderate height, well defined, and extending nearly or quite 

 to the lateral extremities, while it increases rapidly in height, 

 with concave lateral margins toward the beak ; foramen gen- 

 erally higher than wide, with a marginal furrow on each side, 

 and, so far as known, not closed by a deltidium ; mesial sinus 

 narrow, rather deep, sometimes with a small obscure rib along 

 its middle, but more frequently without it; plications on each 

 side of the sinus about five to eight or nine, rather narrow, 

 simple, prominent, and a little rounded; mesial septum of the 

 interior moderately prominent. Dorsal valve with greatest con- 

 vexity near the middle ; beak scarcely projecting beyond the 

 hinge margin, more or less incurved; area very narrow, and 

 incurved with the beak ; mesial fold narrow, not very promi- 

 nent, nor greatly larger than the first plication on either side, 

 most generally rounded, but not infrequently with an obscure 

 sulcus along the middle, near the front ; lateral plications as 

 in the other valve. Entire surface of both valves ornamented 

 with numerous closely crowded, very regularly arranged, sub- 

 imbricating lamelhe of growth, strongly arched in passing over 

 the costa' ; over the whole may also be seen, by the aid of a 

 magnifier, numerous granules, apparently connected with the 

 punctures passing through the shell which are comparatively 

 large and distinct, though regularly arranged." Measure- 

 ments of an average specimen : Entire length, 9 mm. ; width, 

 15 mm. ; convexity, 7 mm. 



Range and distribution : Upper and Lower Coal Measures ; 

 Fort Scott, Bronson, Bourbon county, Thayer, Kansas City, 

 Lawrence, Lecompton, Topeka. 



Not having foreign specimens for comparison, Schuchert is 

 followed in referring our specimens to S. cristata. 



