DEVONIAN SPECIES. 37 



surface with small, obscure, concentric wrinkles and strise of growth (strongest 

 on the ears and sides of the umbonal region), and scattering spine-bases, 

 apparently most numerous on the ears. Dorsal valve somewhat more than 

 semicircular in outline, rather distinctly concave, the greatest concavity be- 

 ing in the central and anterior regions, marked with small, rather regular, 

 concentric wrinkles and strise of growth, with scattering pits corresponding to 

 the positions of the spines of the other valve. 



Length, 0.50 inch; breadth, 0.57 inch; convexity, 0.25 inch. 



In regard to this little shell, I can only say that it seems to be so nearly 

 like European forms referred by good authorities to P. subaculeatus that I 

 have not been able, from the imperfect specimens yet brought in, to be quite 

 sure that it is distinct. ' It is certainly nearly allied to that species, if not the 

 same. As near as can be determined, however, from mere internal casts, it 

 would seem not to have had such distinct tubercles at the bases of the 

 spines as we see represented in most of the published figures of Murchison's 

 species ; though in this respect it appears not to differ from the Russian spec- 

 imens referred by De Verneuil to P. subaculeatus. Still it differs from these 

 and the specimens figured by others, in having more distinct concentric 

 wrinkles, particularly on the dorsal valve. I expect it is probably only a 

 representative species, but have not the necessary specimens to determine 

 the question at present. 



In general appearance, and the absence of any traces of longitudinal 

 strise, it resembles P. pyxidatus of Hall, but differs in being more convex, 

 more symmetrical, and in having smaller ears and stronger concentric 

 wrinkles, particularly on the dorsal valve.* 



Locality and position. — From the dark, silver-bearing limestone, contain- 

 ing Devonian fossils, in White Pine District, Nevada. Colonel Simpson also 

 brought specimens of it from the same rock at latitude 39° 30' N., longi- 

 tude 115° 36' W., in 1860. 



* Nothing sliort of a direct comparison of a good series of authentic European 

 and American specimens can decidedly settle the exact relations of this shell to P. 

 subaculeatus, as well as to several forms described by Professor Hall under the names 

 P. pyxidatus, P. Shumardianus, P. spinulocostatus, P. concentricus, etc. Professor Hall 

 seems now to think these probably all varieties of the one s|)ecies P. Shumardianus ; 

 while European authorities most generally refer all such shells to P. subaculeatus. 



