BRACHIOPODA. 
95 
Surface of the valves smooth, that is, with sharp, concentric striae which 
were never produced into lamellm. 
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, Athyris suh- 
qmdrata, Hall; A. trinuclea. Hall, of the St. Louis limestone; A. Dawsoni, sp. 
nov.,* of the lower Carboniferous beds of Windsor, Nova Scotia; and A. sub- 
tilita, Hall,f of the upper Carboniferous. 
In 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. 
The loop of Seminula suhtilita. Hall. 
(C.) 
In A. suhtilita 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. suhtilita. See Quarterly Journal Geological Society, vol. xix. 
t Athyris suhtilita is a protean species, some of whose variations in general form ai'e illustrated on the 
accompanying plates. One feels at first disinclined to include under the same specific designation the 
broadly ticiform, the narrow elongate, the sinuate, non-sinuate, and trilobed shells which are customarily 
thus referred ; but very abundant material shows the difficulty of separating them. The typical form of the 
species is the elongate shell, broad over the jiallial region, and the exti'eme variations from this type of ex¬ 
terior may have a more or less important faunal or geological value. For examjfie, the most abundant 
representative of the species occurring in the upper Coal Measures about Kansas City, is a naiu-ow, 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. suhquadrata; 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 separate 
from A. suhtilita ; the occurrence of the species at so low an horizon is exceptional, while throughout the 
Coal Measures it is wide and characteristic. 
