LOWEE CARBONIFEROUS FOSSILS. 563 



75 mm. across, with the length about the same. The surface appears to be 

 smooth except for numerous fine growth-lines, and there is also a shallow 

 median sinus. 



The large size of this shell is the only characteristic which is practica- 

 ble for identification, and I know of only two species of Athyroids to which 

 it could possibly be referred, Cliothyris obmaxima and Athyris incrassata. 

 The spinose surface of C. obmaxima puts it out of the question, while. A. 

 incrassata often appears quite smooth. Still I am not at all clear that this 

 specimen is properly referred to A. incrassata. 



I am inclined to believe that Cliothyris obmaxima White 1 would more 

 properly be referred to Athyris incrassata, and that it is perhaps identical 

 with the species under discussion. So far as can be ascertained in the 

 fragmentary and exfoliated condition of White's material, that form did not 

 possess the spinose surface, nor any of the characters of Cliothyris. 



Formation and locality : Madison limestone, northwest slope of Forellen 

 Peak, Teton Range; S. L. Peniield. Burlington age, Burlington, Iowa; 

 Quincy, Illinois; Hannibal, Missouri. 



SEMINULA McCoy, 1844. 



Seminula madisonensis n. sp. 

 PI. LXXI, figs. 2a, 2b, 2c. 



Shell of medium size, subpeiitagonal in outline, somewhat longer than 

 broad. Surface marked by thick lamellose growth-lines, whose edges are 

 smooth, and not prolonged into sheeted or spinose frills as in Athyris (sensu 

 stricto) or Cliothyris. Sometimes marked by fine radiating strise, which are 

 not the result of exfoliation of the fibrous shell structure, but may never- 

 theless be structural. Convexity moderate. 



Ventral valve rather flat, with a shallow angular sinus which can be 

 traced indistinctly nearly to the beak. Beak rather large and not strongly 

 incurved. Dorsal valve broadly angular in cross section ; fold defined for 

 a short distance by shallow converging sulci. The rostral angle of the 

 ventral valve is nearly 90°; that of the dorsal valve is obtuse. 



Length of the type specimen, 19 mm.; width, 16.5 mm.; thickness, 

 11.5 mm. 



'Wheeler's Rept. U. S. Geogr. Surv. W. 100th Merid., Vol. IV, 1877, p. 94, PI. V, fig. 12. 



