254 PALAEONTOLOGY OF ILLINOIS. 



mesial ridge, more or less granular, the granules being often 

 arranged in radiating lines ; visceral scar rather large ; impres- 

 sions of adductor muscles small and not very deep. Surface 

 of both valves, to the unassisted eye, apparently smooth, but 

 showing, under a lens, nearly obsolete traces of radiating striae, 

 crossed by minute concentric lines or wrinkles. Length, 0.42 

 inch ; breadth, 0.49 inch ; convexity, 0.16 inch. 



This species seems to be more nearly allied to C. polita of McCoy {Brit. Pal. 

 Foss, , p. 456, pi. 3 D, Jig. 30), than any other Carboniferous species of the genus 

 with which we are acquainted. It differs, however, in its larger size and more 

 flattened umbo, as well as in showing no traces of the two prominent ridges in 

 the interior of the dorsal valve, mentioned in Mr. Davidson's description 

 of that species {Brit. Carb. Brack.). It also shows, on well preserved speci- 

 mens, faint traces of moderately coarse radiating striae, and none of the spine 

 bases over the surface of the ventral valve, figured by Mr. Davidson. 



Locality and position: Monroe county, Illinois; Keokuk division of the 

 Lower Carboniferous series. 



Genus ATHYRIS, McCoy, 1844.* 



(Carb. Foss. Ireland, p. 146.) 



Athyris planosulcata, Phillips? (sp.) 



PI. 22, fig. 8a, 8b, 8 c, 8 a". 



Spirifer planosulcatus, Phillips, 1836. Geol. Yorksh., vol. ii, p. 220, pi. x, fig. 15. 



Atrypa planosulcata, Soworby, 1840. Min. Conch., vol. vii, p. 15, pi. Dcxyii, fig. 2. 



Atrypa oblonga, Ibid, fig. 3. 



Atrypa planosulcata, de Koninck, 1843. An. Foss., p. 301, pi. xxi, fig. 1 and 2. 



Actinoconchus paradoxus, McCoy, 1844. Synop. Carb. Foss., p. 150, pi. xxi, fig. 6. 



Atrypa obtusa, McCoy, Ibid., pi. xxii, fig. 20. 



Athyris paradoxa, McCoy, 1855. Brit. Pal. Foss., p. 436. 



Athyris parvirostris, Meek and Woethen, Oct. 1860. Pro. Acad. Nat. Sci. Phil., p. 451. 



Shell of medium size, quadrato-subcircular, moderately 

 gibbous, length and breadth nearly equal, sometimes a little 



* It is probable d'Orbigny's later name Spirigera will be retained for this genus; 

 Athyris implying a plain contradiction of fact, since the beak is perforated. 



