"14 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
— 
as high as the dorsal cup. Ambulacra highly elevated, forming broad ridges 
composed of two series of large spiniform plates alternately arranged, which | 
i f pass out from between adjoining orals. Orals spine-bearing, and larger than | 
| | the covering pieces; the posterior one larger than the other four. Inter- | 
| . radial spaces depressed, composed of five nearly flat pieces, arranged in two f 
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. 
mt | Anus at the summit of a short protuberance, opening out laterally, and 
\ ae 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. 
aa Horizon and Locality.— Lower Burlington limestone, Burlington, Iowa, 
| and at the same horizon in New Mexico and Missouri. | 
| Liemarks. — This species, although well characterized, is quite variable in 
| details of structure, and has been described under several names. In some | 
aan specimens, the corrugations upon the plates are more distinct and coarser | 
Hh ' than in others, and sometimes are obsolete near the facet. The form of the 
if | dorsal cup also varies considerably, being in some specimens much deeper 
aaa than in others, and the outer edges of the basals rise slightly above the 
i P| | | 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 
iin 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. 
4 : i Hall’s type of P. striobrachiatus, which has the arms preserved, has the orna- 
a yd | mentation of P. corrugatus, but the form of P. discoideus. In Hall’s P. shu- 
i | tH a mardianus the corrugations are less confluent, and take almost the form of 
mide granules or nodes, less marked, however, than indicated by the figure. In 
i \ | | P. multibrachiatus M. and W., the corrugations do not extend to the median 
q Rid | | . portions of the radials. P. pulchellus is a young specimen of the type of | 
,| eet P. striobrachiatus. 
) ‘ | | | The type of P. shumardianus is in the Illinois State collection, Springfield, 
| a) | those of P. striobrachiatus and P. mwitibrachiatus in the Museum of Com- | 
: | ii a parative Zodlogy; the fate of those of P. discoideus and P corrugatus is 
i unknown. 
