GG PALAEONTOLOGY. 



two sets of spines as in the P. Nchrasccnsis, on which the larger spines were 

 long, strong, and erect. On Dr. Owen's species, the little tubercles, or 

 pi-oniinences sujDporting the spines, are also so much elongated as often 

 almost to 23i"esent the appearance of little costae (especially on internal casts 

 broken from a limestone matrix), which is not the case with the species 

 under consideration. I have now before me many specimens of P. 

 Nchrasccnsis, some of which were collected from the original locality, in 

 Nebraska, while others came from the same horizon in Kansas, Iowa, and 

 Illinois, and none of these attain more than half the size of the species here 

 described, nor are any of them but slightly more than hemispherical; 

 wliile they are all proportionally broader across the umbonal region of the 

 ventral valve. In no instance have I ever seen a specimen of that species 

 so greatly produced in front as we see in this. The most marked differences, 

 however, are to be observed in the internal casts of these two shells, that 

 of P. Nebrascensis scarcely showing any traces of the nmscular scars. 



Among foreign species, our shell is perhaps most nearly represented 

 by some of the narrow, elongated varieties of P. scahriculus, P. pustuhsus, 

 or P. punctatus ; but it seems to me to differ too decidedly and constantly 

 from these to render any detailed comparison necessary. Nevertheless, I 

 expect to see it (as is too often hastily done in similar cases) arranged by 

 some authorities as a synonym of one, or alternately of each, of the 

 above-mentioned species. I am aware that our fig. 2 h, fur instance, 

 jjresents much the appearance of some varieties of P. punctahis, because it 

 is not placed in a posture to show the greatly-produced anterior, as shown 

 in fig. 2 (', nor the strong distant concentric ridges as seen in fig. 2. 

 Those, however, are not the only differences; for our shell is cowsto»2i/^ much 

 narrowc-r than the usual form of P. inmctatus, while its umbonal region has 

 a nuu'li nutre sqiuirish appearance, if I may so term it, owing to the sudden 

 and vertical descent of its sides to the ears. Again, it will be seen to })re- 

 scnt strongly-mnrked diflrcrcnces in the form and relative sizes of the 

 nmscular scars in tlic, ventral valve, by ctmiparing our fig. 2 c with fig. 

 17 of plate xliv of .Mr. Davidson's Monograph of the liritish Carboniferous 

 Hrachiopoda. Equally good distinguishing characters can be pointed out 

 between it and the other species mentioned. 



