]>kede.| Carboniferous Invertebrates. 63 



centric striae, more or less defined both between and upon the 

 radiating stria*, to which latter they impart a neatly crenate ap- 

 pearance." The dental lamelhe are narrow and thin in younger 

 specimens, more rounded and heavier in old ones, extending 

 the entire length of the deltidium and projecting beyond the 

 hinge margin. Muscular impressious of this valve are usually 

 indistinct in young specimens, but are well impressed in the old, 

 thick-shelled individuals, varying from nearly circular to linear- 

 elliptical in outline, in some cases extending two-thirds the 

 distance to the front of the shell, though as a rule it only covers 

 the central portion of the posterior half of the valve. There is 

 a faint mesial ridge in the posterior portion of the brachial 

 valve, extending about half way to the front in old specimens. 

 The muscular scar is nearly circular and marked by longitudinal 

 ridges extending about half way to the front and sides of the 

 valve. Cardinal process moderately prominent, bifid at its apex, 

 and possessing a lateral lobe on each of the supporting plates 

 near their anterior portions. Measurements : Length, 25 mm. ; 

 width, 31 mm. ; length of hinge, 20 mm. 



Range and distribution : Upper and Lower Coal Measures; 

 Fort Scott, Kansas City, Lawrence, Topeka. 



This little depressed or moderately convex shell is very easily 

 distinguished from the other species by its smaller size, moder- 

 ately low hinge area, and thick, or sometimes thin, shell. 



Derbya keokuk. Plate VIII, fig. 13; text fig. 3, E. 



Orthis erenistria YandallandShumard, Cont.Geol. Ky., pp. 19,21, (1817). 

 Orthis keokuk Hall, Geol. Surv. Iowa, i, pt. rr, p. ('40, pi. xix, f. 5, 



(1858); etc. 

 Streptorhynchus keokuk Hall, 2d Ann. Rep. X. V. St. Geol., pi. xli, ft*. 



1-3, (1883). 

 Streptorhynchus en nistria Walcott, Mon. U. S. Geol. Surv., vm, p. 279, 



pi. xviii, f. 14, (1884). 

 Derbya keokuk Hall and Clarke, Pal. X. V., \ m, pt. i, p. 262, pi. xi, ff. 



1-3, 1 1892). 

 .'Orthis umbraculum Owen (non Schlotheim ), Geol. Surv. Wis., Iowa, and 



Minn., pi. v, f. 11, (1852). 

 fOrthis robusta Hall, Geol. Surv. Iowa, i, pt. II, p. 743, pi. xxviii, f. 5, 



(1858). 

 f Streptorhynchus robusta Hall, 2d Ann. Rep. X. V. St. Geol., pi. xl, ft. 



12-17, (1883). 

 />rrf,ya robusta Waagen, Pal. Indica, ser. xiri, i, p. 592, I 1884) ; Hall and 



Clarke, Pal. X. Y.,vill,pt. l, p. 262, pi. x, ft. 12 17, pi. xiB,ff. 7,8,(1892). 



