DIAPHRAGMUS 135 



tion of the species has been largely made. In 1859 McChesney described 

 his P. pileiformis from the Chester limestone, and while examples of the 

 species are less common in this horizon than in the St. Louis limestone, a 

 careful examination of all available specimens has not shown a single 

 character by which they can be distinguished. In the final publication 

 of his paper in 1867, McChesney 1 gives no illustration of the species and 

 makes no mention of it, and the supposition is that he himself abandoned 

 it. The specimen figured by Whitfield under this specific name in 1891 

 and 1895, is an impression of the exterior of the brachial valve and differs 

 in no essential respects from similar specimens of* P. ovatus. White's 

 species P. Icevicostus was described in 1860 from a much lower horizon, 

 the Kinderhook, and his type specimens have never been illustrated. 

 Authentic specimens from the typical locality are at hand, however, and 

 no way has been discovered by which they can be distinguished from the 

 specimens from the higher beds. The P. coroeformis of Swallow 

 has never been illustrated, it has never been recognized since its original 

 definition and was evidently based upon a small example of the common 

 species of our Mississippian faunas. 



From a careful examination of all available material it has been im- 

 possible to detect means of differentiating more than a single species of 

 Productus of the type of P. cora in the Mississippian faunas under con- 

 sideration, and for this species Hall's name, P. ovatus, has priority. The 

 species is a variable one, variations in proportional length and breadth, 

 coarseness of costae, strength of the concentric, wrinkle-like folds, and in 

 the surface spines being common, but in no case have the variations been 

 found to be constant in character. 



Horizon. Kinderhook, Burlington limestone, Keokuk limestone, Salem 

 limestone, St. Louis limestone, Ste. Genevieve limestone, and Chester 

 group. 



Genus DIAPHRAGMUS Girty 



Description. Shell productoid in form, with the external surface of the 

 valves marked by distinct radial costab. Internally the hinge characters 

 of both valves resemble Productus. The diagnostic character of the 

 genus consists of a partition which is an outgrowth from the inner surface 

 of the brachial valve at its geniculation, continuing in nearly the same 

 plane as the visceral portion of the valve to the inner surface of the 

 pedicle valve. Beyond this partition both valves are notably produced, 

 their inner surfaces being nearly or quite in contact. In the fossil speci- 

 mens the visceral surface of the brachial valve continued into the par- 

 tition constitutes a cleavage plane along which the shells are often 

 broken. 



1 Trans. Chicago Acad. ScL, vol. 1 (1867). 



