CARBONIFEROUS SPECIES. 71 



specimens agree pretty closely with certain forms usually regarded in Europe 

 as being only varieties of P. costatus, they never possess the very large 

 costfE and the strong oblique lateral ridge near each ear, seen in the typical 

 vai-iety of that species. Indeed, I have never seen any American shell pre- 

 senting the characters of the typical P. costatus, or nearly approaching it. 

 It is therefore only because the best European authorities include in that 

 species shells with very much smaller costse, and the lateral ridges nearly or 

 quite obsolete (such as that represented by fig.«. 6 and 6 a on plate 4 of Mr. 

 Davidson's Monograph of the Carboniferou.s Brachiopoda of Scotland, first 

 referred by Jfr. Davidson to P. semirdlculatus, but afterward considered by 

 him to be a variety of P. costatus), that we have in this country rci'cri^ il 

 .such forms as that under consideration, even provisional!}', to P. costatus. 

 Generally, our specimens resemble quite closely P. sidcatas of Sowerby; 

 and if it were not for the fact that nearly all European authorities who have 

 investigated the Carboniferous Bracliiopoda agree in viewing the form on 

 which Sowerby proposed to found that name as only a variety of P. ms/«/»s, 

 I should scarcely hesitate to retain the name P. sidcatus for our shell. Our 

 specimens, however, are generally moi'e gibbous and more strongly arched 

 than the European varieties of P. costatus with equally small costse. In 

 most cases, they look much like small examples of P. scmircticulatus ; and, 

 indeed, Mr. Davidson thought that specimens apparently of this shell sent 

 by me to him from the Coal-Measures of Nebraska more probably belonged 

 to that species than to P. costatus ; yet the fact that we find these shells 

 throughout wide areas of country here ranging through a considerable 

 thickness of strata, and often directly associated in the same beds with well- 

 marked examples of P. semiycticuJaius of the usual size and fi)rm, and still 

 never attaining more than half the adult size of that sjjecies, while the}' are 

 always proportionally narrower across the umbonal reg'ion, and more gib- 

 bous and more produced, seems to be an objection to the conclusion that 

 they belong to the same species; especially as these smaller shells are evi- 

 dently, as maybe seen by their much-produced, strongly-arched form, adult 

 specimeus. On the other hand, the fact that they vary comparatively little 

 in their characters, wherever Ave find them, and ticra- nearly approach the 

 typical forms of P. costatus, would seem to )je an eciually good reason for 



