76 BBACHIOPODS. 



hinge margin to be obscurely marked with minute pits for the 

 reception of the crenulations of that of the other valve; teeth 

 small ; muscular impressions long, narrow, separated behind by 

 a short linear mesial ridge, and diverging and extending for- 

 ward beyond the middle of the valve, with a moderately dis- 

 tinct dental ridge along the lateral margin of each ; anterior 

 and lateral regions granulo-striated. Surface of both valves 

 marked by numerous minute, closely arranged radiating striae, 

 about every fourth, fifth or sixth one of which is a little larger 

 and more prominent than those between. ( Meek.) 



Horizon and localities. Lower Silurian, Hudson shales : 

 Louisiana. 



Leptaena mesacosta SHUMARD. 



Leptcena mesacosla Shumard, 1856: Geol. Sur. Missouri, Ann. Rep., p. 

 205, pi. C, fig. 2. 



Shell small, about as wide as long, the hinge-line being 

 much shorter than in L. sericea. 



Horizon and localities. Lower Silurian, Girardeau lime- 

 stone : Cape Girardeau. 



Syntrilasma hemiplicata (HALL). 



Plate xxxix, figs. 8a-d. 

 Spirifer hemiplicatus Hall, 1852: Stanebury's Expd. Gt. Salt Lake, p. 



409, pi. iv, fig. 3a-b. 

 Syntrilasma hemiplicata Meek & Worthen, 1866: Geol. Sur. Illinois, vol. 



II, p. 323, figs. 37a-b. 

 Rhynchonella angulata Geinitz, 1866: Garb, und Dyas in Nebraska, p. 37, 



tab. iii, figs. 1-4. (Not R. angulata Linnaeus, 1767.) 

 Syntrilasma hemiplicata Meek. 1872: U. S. Geol. Sur. Nebraska, p. 177, 



pi. vi, figs. 12a-b. 



Shell subglobose, with several large, rounded plications 

 anteriorly; hinge-line about one-third the greatest width of the 

 valves. Ventral valve not as convex as the dorsal; beak not 

 prominent, somewhat incurved ; cardinal area small, broad, tri- 

 angular; foramen about as wide as high. Dorsal valve much 

 more arched than the other ; beak much incurved ; area nar- 

 row. Surface ornamented with small, radiating lines, which 

 are crossed by lines of growth. 



Horizon and localities. Upper Carboniferous, Upper Coal 

 Measures: Kansas City. 



