92 SUBCAKBONIFEROUS PERIOD. 



in which case the valve has a broad, nearly uniform, convexity ; yet in 

 others the mesial fold is well defined and prominent, especially at the front ; 

 but in all cases it is hardly discernible behind the middle of the shell, even 

 in adult examples. 



Surface of both valves marked by concentric lines, of fine lamellser of 

 growth, and occasionally indications of fine radiating striae are to be seen 

 under a lens. 



Length of an adult specimen of average size, twenty-three millimeters ; 

 extreme breadth, tAventy-nine millimeters ; height, sixteen millimeters. 



This shell is quite different from any other described species of the 

 genus known to occur in American Carboniferous strata. The most 

 nearly allied form with which I am acquainted is an undescribed one in the 

 Subcarboniferous stata'of Franklin County, Iowa, with which our species is 

 probably identical. It somewhat resembles S. incrassatus Hall from the 

 Burlington limestone, but it differs from that species in outline, the less 

 prominence of the ventral beak, the character of the mesial fold and sinus, 

 and in the surface-markings. 



Position and localiti/. — Strata of the Subcarboniferous period ; Mountain 

 Spring, old Mormon road, Nevada. 



Spirigera obmaxima McChesney. 



Plate V, fig. 12. 



Athi/ris ohiiia.vima McOliesuey, 1800, Descr. New Paleozoic Fo.ssils, 80 

 ISjpirii/cra ])cctiiiij'cra Swallow, 1803, Trans. St. Louis Acad. Sci., ii, 88. 



Shell large, broader than long, greatest breadth about midlength ; out- 

 line suboval ; ear-like projections of fibrous shell-substance sometimes seen 

 upon the postero-lateral margins. Ventral valve broadly convex, bearing a 

 broad, shallow mesial sinus upon the front half of old shells, obsolete upon 

 the posterior half; greatest convexity at the Timbonal region ; beak not very 

 prominent, incurved. 



Dorsal valve having its greatest convexity along the median line ; 

 mesial fold indistinctly defined except at the front ; umbonal region not 

 prominent ; beak strongly incurved beneath that of the other valve. 



Breadth, six centimeters or more ; length aljove live centimeters. 



