110 CAEBONIFEROUS PERIOD. 



Surface of both valves, except that of the ears, marked by distinct, 

 more or less unequal, i-ounded, radiating costse, with interspaces of somewhat 

 less than their own width between them; costse generally continuous through 

 a greater part of the length of the shell, but sometimes bifurcating, and 

 occasionally two or more of them may be seen to coalesce and form a single 

 costa of more than ordinary size ; crossing the costse, especially on the 

 posterior half of the shell, and forming indistinct reticulations with them, 

 are concentric wrinkles, more or less numerous. Upon the ventral valve, 

 especially toward the margins, and upon the ears, there are usually scattered 

 strong, more or less perpendicular, spines ; some shells are apparently nearly 

 free from spines, or have only a few of them upon and near the ears. 



This shell is the American representative of the European P. costahis 

 Sowerby, and has been, by European and most American geologists, referred 

 to that species, in the belief that the differences are at most only varietal. Mr. 

 Meek has, however, shown, in his paleontological report for the United 

 States Geological Survey of Nebraska, that there are good reasons for 

 doubting the specific identity of the American with the European shell. If 

 his conclusions are correct, as they are believed to be, the proper name of 

 this shell is P. Portlochianus Norwood and Pratten, these authors having 

 been the first to recognize and publish the American form as distinct from 

 the European. This species as it occurs in American strata is not only 

 a variable one, but some of the varieties show so much constancy that they 

 seem almost worthy of specific separation from the typical forms. One of these 

 varieties was found plentifully near Santa Fe, New Mexico ; but the figures 

 given upon Plate VIII are of such examples as are regarded as typical forms 

 of the species as it occurs in American strata. In P. viminalis White, from 

 the Burlington limestone (Subcarboniferous), the front is more than usually 

 produced, the costse prominent, and more or less fasciculated, but these 

 diff"erences are probably only varietal. 



The geological range of this species is through the strata of the whole 

 Carboniferous period in Illinois, Missouri, Iowa, and Nebraska ; and if P. 

 viminalis is only a variety, as it is now supposed to be, the range is far 

 toward the base of the Subcarboniferous group also. 



Position and localitij. — This is one of the most abundant species in the 



