NATURAL SCIENCES OF PHILADELPHIA. 171 



right valve) is about twice as long as wide, and of a broad lance-ovatc form, 

 most angular at the base. Hinge line scarcely more than one-fourth the 

 greatest length of the shell, measuring obliquely from the anterior extremity 

 to the posterior basal margin, and ranging at an angle of about 45° to the 

 longer axis of the valves. Beaks very small, incurved, and depressed almost 

 to the cardinal margin ; located nearlj- over the anterior edge. Surface with 

 concentric striie obscure ; radiating costte of the posterior dorsal region very 

 faintly marked, and broader than the slight furrows between, while very 

 obscure traces of fine longitudinal strife may be seen on some of them ; radia- 

 ting cost88 of the anterior, fine, and rather sharply defined on the gaping edge, 

 back of which a few larger obscure ribs may be seen, the posterior one of 

 which is larger than the others, and curves down from the anterior side of 

 the beak so as to intersect the margin of the valve a little below the lower 

 end of the hiatus. 



Length, 2 inches ; height, measuring at right angles to the greatest length, 

 1-48 inch ; convexity of the left valve, 0-50 inch ; length of anterior hiatus, 

 0-G3 inch ; breadth of do. in same, 0-J6 inch. 



This shell differs so widelj' from all others known to us from our Carboni- 

 ferous rocks, that acomparison is unnecessary. Indeed we know of no other 

 form liable to be confounded with it, from rocks of any age. 



Locality and position. — West Pecan Creek, Grundy County, from the lower 

 part of the Coal Measures. 



Genus ALLORISMA, King. 

 Allorisma costata, M. and W. 



Shell under medium size, longitudinally oblong, the length being more than 

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

 anterior margin rather short, closed and narrowly rounded; basal margin 

 forming a long nearly semi-elliptic curve, with a very slight sinuosity in front 

 of the middle ; posterior side compressed ; 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 aVjove the uubonal ridge; cardinal margin equaling 

 about two-thirds the entire length of the shell, very nearly straiglit, and inflec- 

 ted so as to form a narrow or lance-linear corselet, extending its whole length; 

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

 more than one-sixth the length of the valves behind the anterior extremity; 

 lunule well defined and lance-ovate in form. Surface ornamented by about 

 twenty-five very regularly arranged, distinctly elevated concrntric cost*, 

 which commence near the lunule, and extend backward parallel to the base, to 

 the well defined, angular ural)onal ridge, leading from the beaks to the 

 posterior basal extremity, at which ridge they become suddenly obsolete, or 

 very nearly so, being mainly represented on the more compress 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 of the usual minute surface gran- 

 ules appear to be visible in some of the moulds left in the matrix. 



Length, about 1-20 inch; height, 0-53 inch ; convexity, 0-44 inch. 



This is a very neat, elegant species, of the type A. ekffans^ Kiug, and A. 

 Geinitzii, Meek, (^=E. elegans, Geinitz,* not King). It is a more slender species, 

 however, with much more sharply elevated, and more regularly disposed costae 

 than the former ; while it will also be readily distinguished from the latter by 

 its costae, and much more depressed umbones. wider (higher) posterior ex- 

 tremity, &c. The regularity and prominence of its concentric cost;ie, and their 

 very abrupt termination along the umbonal ridge or carina, are remarkable 



* Carbon f. und Dyas iu Nebraska. 



1869.] 



