344 PALEONTOLOGY OF OHIO. 



Genus ALLORISMA, King, 1844. 



(Ann. Mag. Nat. Hist., XIV., 316.) 



Allokisma costata, M. and W. 



Plate 19, figs. 6n, b. 



AUorisma costata, Meek and Worthen (1869) ; Proceed. Acad. Nat. Sci., Philad., XXI 

 171, and (1873) Eeport Geol. Survey of Illinois, V., 585, pi. 26, fig. 15. 



Shell small, or under medium size, longitudinally oblong, or trape- 

 zoidal, with length a little more than twice the height, moderately convex 

 in the central and umbonal regions ; anterior margin short, closed and 

 rounded; basal margin forming a long, semielliptic curve; posterior side 

 compressed, apparently a little gaping, somewhat obliquely truncated 

 above, and connecting with the base below, so as to form a more or less 

 defined angle at the termination of the oblique, posterior umbonal cari- 

 nae; posterior dorsal region above the umbonal carinas compressed; car- 

 dinal margin equaling more than half the length of the valves, nearly 

 straight, and sl'ghtly inflected, so as to form a very narrow, obscurely 

 defined false aica or corselet; beaks rising a little above the cardinal 

 margin, and located near the anterior end; lunule small but well defined, 

 and lanc(.-oval in form. Surface ornamented by very regular, sharply 

 raised, ccii centric costas, separated by wider, rounded furrows, and all 

 ending vtjry abruptly behind, at the equally angular umbonal carinee. 



Length, 0.63 inch; height, 0.32 inch; convexity, about 0.20 inch. 



The specimen from which our figures were drawn is not quite so com- 

 plete on the posterior margin as fig. 6a would indicate, this margin being 

 a little broken, so that the figure does not give its exact original outline. 

 The surface of the posterior dorsal region is also not in a condition to 

 show the lines of growth as represented on the figures, while fig. 66 is 

 defective in not showing the narrow, obscure corselet, which, however, 

 is nearly obliterated in the specimen by compression. 



On comparing the figures on plate 19 with that of the large, more 

 nearly perfect typical specimen given on plate 26, Vol. V., of the Illinois 

 Report, the latter will be seen to difi^er in having its posterior margin 

 bitruncated, and a second oblique ridge on the compressed space above 

 the umbonal angle. This difl^erence, however, seems to be mainly, if not 

 entirely, due to the imperfection and smaller size of the Ohio specimen, 

 which, as above stated, has the posterior margin not entirely perfect in 



