298 PALAEONTOLOGY OF ILLINOIS. 



the beak, which is less prominent. Its hinge line is also more extended, its 

 surface striae finer and more regular, and its dorsal valve much more deeply 

 concave. This valve likewise always differs in being entirely destitute of the 

 concentric ridge so characteristic of P. elegans. 



It differs from P. longispinus, in always having much finer and more regu- 

 lar striae, with less distinct concentric wrinkles; while its dorsal valve is 

 uniformly more deeply concave, particularly in the middle. 



It is much more closely allied to Productus Flemingii, of Sowerby — a variety 

 of P. longispinus — than to the typical forms of that species. On comparison, 

 however, with authentic specimens of P. Flemingii, sent from Scotland by Mr. 

 Thomas Davidson, we find our shell has the umbo of its ventral valve more 

 ventricose, and more incurved, while its dorsal valve is always more deeply 

 concave, particularly near the beak and in the middle. 



Locality and position: Chester, Illinois; Chester group of the Lower Car- 

 boniferous series. 



Genus SPIRIFER, Sowerby, 1815. 



(Min. Conch., ii, p. 42.) 



Subgenus MARTINIA, McCoy, 1844. 

 Spirifer glaber, var. contractus, M. and W. 



PI. 23, fig. 5a, 5 6. 



Conchyliolithus anomites glaber, Martin, 1809. Petref. Derb., pi. xlviii, fig. 9, 10. 



Spirifer glaber, Sowerby, 1820. Min. Conch., vol. iii, p. 123, pi. cclxix, fig. 1. 



Spirifer obtusus, Sowerby, 1820. Ibid, fig. 2. 



Spirifer oblatus, Sowerby, 1820. Ibid, pi. cclxviii. 



Trigonotreta oblata, Bronn, 1836. Leth. Geog., p. 81, pi. ii, fig. 16. 



Spirifera linguifera, Phillips, 1836. Geol. Yorksh., vol. ii, p. 219, pi. x, fig. 4. 



Spirifera symmetrica, Phillips, 1836. Ibid, fig. 13. 



Spirifera discora? Phillips, 1836. Ibid, fig. 9. 



Spirifer l-xvigatus, von Buch, 1840. Mem. Soc. Geol. France, vol. iv, p. 198. 



Spirifer obtusus, and S. oblatus, McCoy, 1844. Synops. Carb. Foss. Ireland, p. 139. 



Shell rather under medium size, quadratp-subcircular in out- 

 line, becoming moderately gibbous with age ; length and breadth 

 nearly equal ; sides rounded ; hinge short, or scarcely equaling 

 half the breadth of the valves near the middle. Dorsal valve 

 much more compressed than the other, most convex along the 



