112 CARBONIFEROUS PERIOD. 



defined mesial sinus extends from near the umbonal region to the front, 

 giving the shell an indistinctly-bilobed appearance. Dorsal valve flattened 

 in the visceral region, bent abruptly upward at the sides and front ; beak 

 flattened or slightly concave. 



Surface of both valves marked by numerous coarse, rounded striee, or 

 small costae, which are crossed in the visceral region by somewhat regular 

 concentric wrinkles of nearly uniform size, giving that part of the sliell a 

 semireticulated appearance, which is more distinct in some examples than in 

 other>s ; more or less numerous, strong, erect spines are scattered upon the 

 ventral valve, generally arisuig from the costse upon the body of the shell, 

 and fi-om the strong wrinkles upon the ears, upon which latter part they 

 are usually most numerous. 



Length of a full average-sized specimen, measured in a straight line 

 from hinge to front, four and a half centimeters ; width, six centimeters. 



The foregoing synonymy, copied largely from the works of Davidson 

 and Meek, shows that this shell presents such variations, both in America 

 and Europe, that it has been described under a variety of specific names 

 by paleontologists of both hemispheres. The differences between the varie- 

 ties thus described under diff'erent specific names are, in some cases at least, 

 really very great; but, so far as my own observation has extended, there is 

 such a want of constancy in these varieties, and such a lack of definite 

 specific characters by which they may be clearly separated, that I am dis- 

 posed to refer them all to one species, with the possible exception of P. Ivesi 

 Newbeny. 



Viewing the species in this light, it is one of the most remarkable of all 

 known fossil shells, not only for its great variation, but for its wide geo- 

 graphical distribution and great geological range. It is now known as a 

 common fossil in the Carboniferous strata of Europe, India, South America, 

 and North America. It is found in the Keokuk limestone of the Sub- 

 carboniferous period, in Illinois, Iowa, and Missouri, and also ranges thi'ough 

 the whole series of strata of the Carboniferous period of those States as well 

 as in Kansas and Nebraska. The localities given below make a still further 

 addition to its known geographical range ; but most of the examples in the 

 collections are of the variety P. Ivesii. 



