LOWER CARBONIFEROUS FOSSILS. 565 



valve more highly arched both longitudinally and transversely; sinus not 

 so distinct. 



Length, 11 mm.; width, 9 mm. 



Formation and locality: Madison limestone, limestone bluff north of 

 Little Sunlight Creek, Absaroka Range, 600 feet above the stream; Arnold 

 Hague. 



Seminula humilis n. sp. 



PI. LXXI, figs. 6«, 6&, 6c. 



Shell moderately gibbous, small, circular. Surface smooth, except 

 for a few growth-lines. 



Ventral valve nearly circular, but for the beak, which is rather large 

 and strongly incurved over the other valve, completely concealing the 

 foramen, which appears only where broken back through the rostral shell. 



Dorsal valve circular in outline, moderately curved; beak somewhat 

 prominent by reason of a slight flattening on either side. The dorsal valve 

 has an ir distinct fold and the ventral valve an insignificant sinus, which are 

 perceptible only by a sinuosity in their anterior margins. 



Length of a medium-sized specimen, 15 mm.; breadth, 14.5 mm.; thick- 

 ness, 9 mm. Length of a somewhat smaller individual, 12.5 mm.; breadth, . 

 11.5 mm.; thickness, 7.5 mm. 



Were it not that S. wasatchensis is an Upper Carboniferous form, 

 while this one is found in the lower beds of the Lower Carboniferous, I 

 would have unhesitatingly referred it to White's species. The great anterior 

 thickening of the shell, which appears to individualize the latter type, is 

 due to old age, and can not be considered a specific character. Otherwise, 

 if occurring in the same beds, the two forms could scarcely be distinguished. 



Seminula humilis also approaches Spirigera formosa and Sp. euzona, both 

 of Swallow. These forms, however, are marked by a high fold and corre- 

 sponding deep sinus, while Sp. formosa is said to have a punctate shell, 

 and radiating striae when exfoliated (fibrous shell structure!). These 

 characters are decisive in separating Swallow's species from the one in 

 question. 



Formation and locality: Madison limestone, Hunter Peak, Absaroka 

 Range; T. A. Jaggar. Crowfoot Ridge, Gallatin Range, lower and upper 

 part of bed 27, bed 28, bed 31 ; J. P. Iddings and W. H. Weed. Stinking- 

 water Valley, below mouth of the canyon, Absaroka Range; Arnold. 



