170 University Geological Survey of Kansas. 



seen, 4.81 inches ; height from the ventral to the dorsal mar- 

 gins, near middle, 1.76 inches ; convexity, 1.57 inches." 



Range and distribution : Upper Coal Measures ; Westport 

 (Mo.), Kansas City, Mont Ida (Anderson county), Lawrence, 

 Lecompton, Topeka, Elmont, Grand Summit. 



This species is distinguished, on account of its larger and 

 more graceful form, from either of the preceding. 



Allorisma costatum. Plate XX, fig. 12. 



Allorisma costata Meek and Worthen, Proc. Acad. Nat. Sci. Phil. 1869, 

 p. 171; Geol. Surv. 111., v, p. 585, pi. xxvr, f. 15, (1873). 



Meek and Worthen's description : " Shell under medium size, 

 longitudinally oblong, the length being more than twice the 

 height, very thin, rather convex in the central umbonal regions ; 

 anterior margin rather short, closed and narrowly rounded ; 

 basal margin forming a long, rather semielliptic curve, with a 

 very slight sinuosity in front of the middle ; posterior side com- 

 pressed, but apparently a little gaping and distinctly truncated 

 nearly vertically from the base about half way up, and thence 

 a little obliquely forward and upward to the dorsal margin ; 

 posterior dorsal region compressed above the umbonal ridge ; 

 cardinal margin equaling about two-thirds the entire length 

 of the shell, very nearly straight, and inflected so as to form a 

 narrow or lance-linear corselet, extending over its whole length ; 

 beaks convex, rising a little above the cardinal margin, and 

 placed slightly more than one-sixth the length of the valve be- 

 hind the anterior extremity ; lunule well defined and lance- 

 ovate in form. Surface ornamented by about twenty-five very 

 regularly arranged, distinctly elevated, concentric costse, which 

 commence near the lunule and extend backward parallel to the 

 base, to the well-defined, angular umbonal ridge leading from 

 the beaks to the posterior basal extremity, at which ridge they 

 become suddenly obsolete, or very nearly so, being mainly rep- 

 resented on the more compressed posterior dorsal region by 

 distinct lines of growth, which are crossed on the middle of 

 this area by a second oblique linear ridge extending from the 

 beaks to the middle of the posterior margin. Some indications 



