186 PALAEONTOLOGY OF ILLINOIS. 



FAMILY PLATYCRINID.E. 



GENUS PLATYCRINUS. 

 PLATYCKINUS SYMMETRKTS (nov. sp.) W. & Sp. 



PL XV, Fig. 8. Specimen with arms, column and root. 



This species is of the type of P. burlingtonensis O. & Sh. (Geol. 

 Surv. Wisconsin, Iowa and Minnesota, 1852, p. 589,) and may 

 possibly be identical with some one of the various smooth species 

 that have been described from the Burlington limestone. Several 

 of these species were based upon specimens in which neither the 

 vault nor the arms were preserved, and from the meagre informa- 

 tion we obtain from mere descriptions of the basal and first radial 

 plates, it is not easy to identify them satisfactorily. Our species 

 differs from the P. burlingtonensis, as described by Owen and 

 Shumard, in having a proportionally smaller basal disk, longer 

 and narrower first radials; and from specimens identified by the 

 Burlington paleontologists as representing that species, it differs 

 in the form and structure of the ventral side, which in all Burling- 

 ton specimens is considerably elevated above the arm bases, and 

 occupied chiefly by the extremely large summit plates and the 

 base of a large anal tube. In P. symmetricus the ventral covering- 

 is low, to a large extent formed by rows of rather prominent cov- 

 ering plates, and the anus is represented by a simple subcentral 

 opening. It agrees with P. hurlingtonensis, however, in having 

 thirty arms. From P. pileiformis Hall, it differs in the vault 

 structure, in its truncate base, and, besides, that species, as shown 

 in our collection, has but twenty, and much heavier arms. The 

 new species bears a strong resemblance to an undescribed species 

 from the Lower Burlington limestone of New Mexico, not only 

 in the shape of the radials, but also in the proportionate size of 

 the basal disk, and the outline of the calyx generally. It agrees 



