DIELASMA 267 



the name D. form&sa, and most of the loop bearing shells of the Mississip- 

 pian faunas commonly have been referred, in recent years, either to this 

 species or to Girtyella turgida. The species is a well-defined one, character- 

 ized by the ovate outline of the shell, rounded in front, with neither fold 

 or sinus in either valve. The shell described by Hall and Clarke as D. 

 obovdta, and said to be from the Pennsylvanian is evidently a large example 

 of this species. The original label with the type specimen is marked 

 " Carboniferous Limestone, Kentucky," with no suggestion that it came 

 from the Pennsylvanian. Furthermore, the lithologic character of the 

 specimen is identical with specimens from the Salem limestone of southern 

 Indiana or Kentucky, the typical horizon for Dielasma formosa. 

 Horizon. Salem limestone. 



DIELASMA ILLINOISENSIS n. sp. 

 Plate XXXIII, Figs. 18-20 



Description. Shell below medium size, elongate subovate in outline, the 

 greatest width near the mid-length, the anterior margin rounded. The 

 dimensions of a nearly perfect specimen are: length of pedicle valve 

 21 mm., length of brachial valve 18.8 mm., greatest width 12.7 mm., thick- 

 ness 10 mm. 



Pedicle valve rather strongly convex, the greatest convexity posterior 

 to the middle, the surface arcuate from beak to front along the median 

 line with the curvature increasingly convex posteriorly, the convexity at 

 first moderate from the median line towards the postero-lateral margins, 

 curving more abruptly as it approaches the margin and becoming inflected 

 to the cardinal extremities, the curvature more gentle towards the antero- 

 lateral margins; mesial sinus of moderate width and depth, rounded in 

 the bottom, originating near or a little posterior to the middle of the 

 valve; the beak prominent and closely incurved, the foramen large, sub- 

 circular or subovate in outline, encroaching wholly upon the umbonal 

 portion of the valve, the deltidial plates hidden by the incurvature of 

 the beak. Internally the dental lamellae are well developed and of mod- 

 erate length. 



FIG. 30. A series of seven cross-sections of the rostral portion of the shell of 

 Dielasma illinoisensis (X 2^), showing the dental lamellae of the pedicle valve, 

 and the crural lamellae developed independently from the socket plates. 



The brachial valve less convex than the pedicle, gently convex longi- 

 tudinally from beak to front, the curvature usually a little greater near 



