128 LA.MELLIBRANCHS. 



Allorisma costata MEEK & WORTHEN. 

 Plate xlvi, fig. 12. 



Allorisma costata Meek & Worthen, 1869: Proc. Acad. Nat. Sci., Phila.. 



p. 171. 



Allorisma costata Meek & Worthen, 1873: Geol. Sur. Illinois, vol. V, p. 

 585, pi. xx vi, fig. 15. 



Shell small, elongate, length from two to three times the 

 height: thin, convex in the umbonal regions ; anterior margin 

 short, evenly rounded ; posterior end compressed, truncated. 

 Surface ornamented by sharp, distant concentric ridges, which 

 extend backward to the well-defined umbonal carinse. 



Horizon and localities Upper Carboniferous, Upper Goal 

 Measures: Kansas City. 



Allorisma topekaensisfSnuMARD). 



Leptodomus topekaensis Shumard, 1858: Trans. St. Louis Acad. Sci., vol. 

 1, p. 208. 



Very closely related to A. granosum, and perhaps identical 

 with that species. 



Horizon and localities. Lower Carboniferous, Upper Coal 

 Measures : Kansas City. 



Allorisma granosum (SHUMARD.). 



Leptodomus granosus Shumard, 1858: Trans. St. Louis Acad. Sci., vol. I, 



p. 207. 

 Allorisma granosum Meek, 1872: U. S. Geol. Sur. Nebraska, p. 220, pL 



ii,fig. 8. 



Shell very thin ; approaching an irregularly oblong form, the 

 length being less than twice the height; very convex, the most 

 gibbous part being near the middle of the valves ; beaks 

 prominent, incurved, somewhat flattened on the outside, and 

 placed about half way between the middle and the front. Dor- 

 sal margin straight behind the beaks and nearly parallel to the 

 general outline of the base, inflected so as to form a distinct, 

 flattened, lanceolate, lunule-like area, bounded on each side by 

 a well-defined, sub-angular ridge; posterior side nearly or 

 quite closed, obliquely truncated, with sometimes a faint sinu- 

 osity near the middle ; anterior side rather abruptly sloping 

 forward, straightened above, and rounding into the base below, 



