714 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



as higli as the dorsal cnp. Ambulacra highly elevated, forming broad ridges 

 composed of two series of large spiniform plates alternately arranged, which 

 pass out from between adjoining orals. Orals spine-bearing, and larger than 

 the covering pieces ; the posterior one larger than the other four. Tnter- 

 radial spaces depressed, composed of five nearly flat pieces, arranged in two 

 rows, all of which are interambulacral. The middle plate of the first row 

 very large ; the two at the sides as long as the other, but much narrower. 

 Anus at the summit of a short protuberance, opening out laterally, and 

 placed between the plates of the first and second row, which together with 

 the four smaller orals and the upper plate of the two posterior ambulacra 

 form a ring around the larger orals, giving to the latter a strictly central 

 position. Column twisted ; the proximal joint circular, the succeeding ones 

 elliptic ; and increasing in length downward. 



Horizon and Locality. — Lower Burlington limestone, Burlington, Iowa, 

 and at the same horizon in New Mexico and Missouri. 



Remarks. — This species, although well characterized, is quite variable in 

 details of structure, and has been described under several names. In some 

 specimens, the corrugations upon the plates are more distinct and coarser 

 than in others, and sometimes are obsolete near the facet. The form of the 

 dorsal cup also varies considerably, being in some specimens much deeper 

 than in others, and the outer edges of the basals rise slightly above the 

 plane of the radials. Owen and Shumard's figure of P. discoideus repre- 

 sents an extremely flat specimen, and its corrugations are very coarse. In 

 their P. corrugatus, on the other hand, the markings are fine, and the dorsal 

 cup considerably deeper, the two representing the extremes of the species. 

 Hall's type of P. striohracJiiatus, which has the arms preserved, has the orna- 

 mentation of P. corrugatus, but the form of P. discoideiis. In Hall's P. slni- 

 mardianus the corrugations are less confluent, and take almost the form of 

 granules or nodes, less marked, however, than indicated by the figure. In 

 P. midtibracliiatus M. and W., the corrugations do not extend to the median 

 portions of the radials. P. pulclidlus is a young specimen of the type of 

 P. striohracliiaius. 



The type of P. shumardianiis is in the Illinois State collection, Springfield, 

 those of P. striohraeliiatus and P. midtibracliiatits in the Museum of Com- 

 parative Zoology ; the fate of those of P. discoideus and P. corrugatus is 

 unknown. 



