BEACHIOPODS. 93 



Terebratula subtilita Marcou, 1858: Geol. N. A., p. 52, pi. vi, fig. 9. 

 Spirigera subtilita Meek & Hayden, 1859: Proc. Acad. Nat. Sci., Phila., 



p. 20. 

 Athyrw differentia McChesney, 1859: Desc. New Sp. FOBS. Pal. Bocks 



Western States. 

 Spirigera charitontnsis Swallow, 1860: Traos. St. Louis Acad. Sci., vol. 



I, p. 651. 



Spirigera hawni Swallow, 1860 : Trans. St. Louis Acad. Sci.. vol. 1, p. 652. 



Athyris subtilita dewberry, 1861: Ives' Exped. Colorado River, Paleon- 

 tology, p. 126 



Spirigera singletoni Swallow, 1863: Trans. St. Louis Acad. Sci., vol. II, 

 p. 87. 



Spirigera capnt-serpentii Swallow, 1863 : Trans. St. Louis Acad. Sci., vol. 



II, p. 90. 



Athyris subtilita Geinitz, 1866: Garb, und Dyas in Nebraska, p. 40, tab. 



iii, figs. 7-9. 

 Athyris subtilita Meek, 1872: U. S. Geol. Sur. Nebraska, p. 180, pi. i, 



fig. 12, pi. v, fig. 8, pi. viii. fig. 4. 

 Athyris subtilita Meek & Worthen, 1873 : Geol. Sur. Illinois, vol. V, p. 570, 



pi. xxv, fig. 14. 

 Spirigera subtilita White, 1885: U. S. Geol. Sur. w. 100 Merid., vol IV, 



p. 141, pi. x, figs. 6a-c. 

 Athyris subtilita Meek, 1877 : U. S. Geol. Exp. 40 Par., vol. IV, p. 83, pi. 



viii, figs. 6, 6a. 

 Athyris subtilita White, 1884 : Geol. Sur. Indiana, 13th Ann. Kept , p. 136, 



pi. xxxv, figs. 6-9. 

 Athyns subtilita Keyes, 1888 : Proc. Acad. Nat. Sci., Phila., p. 231. 



Shell rather small, sublentieular to subglobose. Ventral 

 valve regularly arched and usually slightly more convex than 

 opposite one ; beak quite prominent, rounded, incurved and 

 truncated by the circular foramen ; median sinus obsolete in 

 young specimens, but often quite prominent in older individ- 

 uals. Dorsal valve convex, with a well-marked mesial fold in 

 adult specimens; beak not prominent. Surface of valves glab- 

 rate, with occasional lines of growth, which, in large specimens, 

 are more or less imbricated. 



Horizon and localities. Upper Carboniferous, Coal Meas- 

 ures : Kansas City, Lexington, Clinton (Henry county). 



The name Terebratula argentea was proposed by Shepard 

 more than half a century ago for a shell from the Coal Measures ' 

 of LaSalle, Illinois, in all respects apparently identical with the 

 form described by Hall fourteen years later. Although Shep- 

 ard's diagnosis is quite brief, and his two figures rather crude, 



