CARBONIFEROUS PERIOD. 137 



Trigonotrcta oblata Brouu, 1S3G, Letbtea Geognostica, i, 81. 



Spirifer glabcr Phillips, 1836, Geology of Yorkshire, ii, 219. 



Spirifer Iwvigatus von Buch, 1840, Mem. de la Soc. G6ol. de France, iv, 198. 



Spirifer glaher de Kouinck, 1844, Animaux Fossiles de la Belgique, 207. 



Martinia (jlahra McCoy, 1844, Synopsis Carb. Fossils of Ireland, 139. 



Spirifera glabra McCoy, 1855. British Palaeozoic Fossils, 428. 



Spirifera glabra Davidson, 1857, Monog. British Carb. Brachiopoda, 59. 



Spirifer glaber var. contracta Meek and Worthen, 186C, Geol. Siirv. of Illinois, ii, 298. 



Shell reaching about medium size, rotund, gibbous, or becoming much 

 inflated with age ; length and breadth usually about equal ; hinge-line short, 

 not quite equal to half the transverse diameter of the shell. Dorsal valve 

 less capacious than the other, subcircular, broadly rounded in front, most 

 convex near the beak; the beak is small, slightly prominent, and projects 

 trifle over the cardinal margin. Ventral valve capacious, arcuate; umbo 

 extended much behind the hinge-line, especially in adult shells ; mesial sinvts 

 absent or obsolete ; beak prominent, pointed, and strongly incurved ; area 

 narrow laterally, moderately high, concave, lateral borders obscurely 

 defined ; foramen comparatively large, occupying the greater part of the 

 area. 



Surface apparently smooth, but under a lens fine concentric lines are 

 seen, which appear to have been the seat of concentric rows of very minute 

 set£e ; concentric folds are also sometimes present, especially toward the 

 margins of old shells. 



Length and breadth of the largest specimen in the collection, each six 

 millimeters ; height, twenty millimeters ; but this is considerably larger than 

 tlie average size of the others, and the height is proportionally greater. 



The type-specimens of this variety of S. glaher were obtained by Meek 

 and Worthen from the Chester limestone (Subcarboniferous) at the town of 

 Chester, Illinois. Our shell agrees so exactly with it that I feel compelled 

 to refer it to that variety notwithstanding the fact that it was obtained from 

 strata of another and later period. The principal observable difierence 

 between ours and Meek and Worthen's type-specimens seems to be a nearer 

 approach in their shell to a defined mesial sinus than in ' ours, but this 

 difference may be no more than an individual one. If it is not referable to 

 this variety, I am not able at present to refer it to any other than the original 

 Eiu'opean species. It especially resembles a variety from Yorkshire, 



