BRACHIOPODA. 



95 



Surface of the valves smooth, that is, with sharp, concentric striae which 

 were never produced into lamellag. 



The shells which constitute this group were apparently confined to the faunas 

 of the Carboniferous age. The number of species in American faunas is not 

 great, but we have now a pretty thorough understanding of four, Alhyris sub- 

 quadrafa. Hall; A. trinuclea. Hall, of the St. Louis limestone; A. Daivsoni, sp. 

 nov.,* of the lower Carboniferous beds of Windsor, Nova Scotia; and A. sub- 

 tilita, Hall,f of the upper Carboniferous. 



Tn all these species we now know the structure of the loop, and though in 

 each it has a characteristic form, its variations are not of great significance. 



FIG. 67. 



The loop of Seminula subtilita, Hali. 



(C.) 



In A. subtilita its position is more posterior than in the other species, the 

 umbonal blades of the primary and accessory lamellae are broader, the saddle 



* Identified by Davidson as A. subtilita. See Quarterly Journal Geolog-ical Society, vol. xix. 



t Athyris subtilita is a protean species, some of whose variations in g-eneral form are illustrated en the 

 accompanying- plates. One feels at tirst disinclined to include under the same sjiecific designation the 

 broadly ticiform, the narrow elongate, the sinuate, non-sinuate, and trilobed shells which are customarily 

 thus referred ; but very abundant mateiial shows the difficulty of separating them. The typical form of the 

 species is the elongate shell, broad over the pallial region, and the extreme variations from this type of ex- 

 terior m.ay have a more or less important faunal or geological value. For example, the most abundant 

 representative of the species occurring in the upper Coal Measures about Kansas City, is a narrow, elongate, 

 slightly sinuate shell, one extreme of variation ; again, we have been supplied by Professor S. Calvin with 

 a series of specimens from Winterset, Iowa, some of which are as deeply trilobate on the anterior margins as 

 extreme forms of A. siibquadiata ; in both instances these variations are found to pass into the typical form 

 of the species by insensible gradations, and as far as known there is little variability in the structure of the 

 interior. In the St. Louis limestone at Pella, Iowa, there occurs a form which it is impossible to sejiarale 

 from A. svhtillta ; the occurrence of the species at so low an horizon is exceptional, while throughout the 

 Coal Measures it is wide and characteristic. 



