OF ARTS AND SCIENCES. 299 



P. BuRLiNGTONENSis, Owen and Shuraard, 1. c. Geol. Survey of 

 Wisconsin, Iowa, and Minnesota, p. 589, PL 5 A, fig. 5. 

 Suhcarhoniferous. Burlington limestone. 



P. CANICULATUS, Hall, lowa, Vol. 1, p. 539. Suhcarhoniferous. 

 Burlington limestone. 



P. CAVUS, Hall, Iowa, Vol. 1, p. 527, PI. 8, fig. 1, a, b. Compare 

 P. corrugatus, Owen and Shumard, Geol. Survey of Iowa, 

 Wisconsin, and Minnesota, p. 589, PI. 5 A, fig. 2, a - d. Suh- 

 carhoniferous. Burlington limestone. 



P. CORRUGATUS, Owen and Shumard, 1. c. See P. cavus. Suh- 

 carhoniferous. Burlington limestone. 



P. DiscoiDEUS, Owen and Shumard, Journ. Acad. Nat. Sci. Philad., 

 New Series, Vol. 2 ; Geol. Survey of Wisconsin, Iowa, and 

 Minnesota, p. 581, PI. 5 A, fig. 1 ; Hall, Iowa, Vol. 1, p. 535, 

 PI. 8, fig. 8, a, b. Suhcarhoniferous. Burlington limestone. 



P. EXSERTUS, Hall, Iowa, Vol. 1, p. 539. Suhcarhoniferous. Bur- 

 lington limestone. 



P.NODOBRACHIATUS, Hall, 1. c. p. 542, diagram. Suhcarhonifer- 

 ous. This may be the young of some known species. 



P. NODULOSUS, Hall, 1. c. p. 541. Suhcarhoniferous. Burlington 

 limestone. 



P. NUCLEiPORMis, Hall, lowa. Vol. 1, p. 540, diagram. Suhcar- 

 honiferous. Burlington limestone. 



P. piLEiFORMis, Hall, Iowa, Vol. 1, p. 529, PI. 8, fig. 3, a, b. Suh- 

 carhoniferous. Burlington limestone. 



P. PLANUS, Owen and Shumard, Journ. Acad. Nat. Sci. Philad., 

 New Series, Vol. 2, 1851 ; Geol. Survey of Wisconsin, Iowa, 

 and Minnesota, p. 587, PI. 5 A, fig. 4 ; Hall, Iowa, Vol. 1, 

 p. 533, PI. 8, fig. 6, a, b. Suhcarhoniferous. Burlington 

 limestone. 



P. pcECiLLiFORMis, Hall, lowa, Vol. 1, p. 528, PI. 8, fig. 2, a, b. 

 Suhcarhoniferous. Burlington limestone. 



P. Saffordi, Troost, MSS. ; Hall, Iowa, Vol. 1, p. 634, PI. 18, 

 fig. 5, 6. Suhcarhoniferous. Keokuk limestone. 



P. Sar^, Hall, Iowa, Vol. 1, p. 673, PI. 17, fig. 4. Suhcarhon- 

 iferous. Keokuk limestone. 



P. SCULPTUS, Hall, Iowa, Vol. 1, p. 536, PI. 8, fig. 2. Suhcarhon- 

 iferous. Burlington limestone. 



