168 PALAEONTOLOGY OF ILLINOIS. 



From what is now known, this genus may be described as 

 follows : Basal pieces two, hexagonal, and forming when uni- 

 ted, an obconic or more or less depressed cup. The five first 

 radial pieces very large, resting directly upon the base, two 

 upon each basal piece, and the anterior one in a notch at one 

 end of the suture between the latter. Succeeding radial pieces 

 one to three in each ray,* very small, the last supporting the 

 arms, which in the typical forms of the genus, are slender and 

 more or less frequently bifurcating. First anal piece nearly 

 or quite as large as the first radial, and resting upon the base 

 between them, in a notch at one end of the suture between the 

 basals; the remaining anals small, and not very distinct from 

 the vault pieces. Interradials four or five, very small, and 

 similar to the vault pieces, resting in small notches at the 

 superior lateral angles of the first radials. 



The vault of D. pocillum, Hall, is described as consisting of few plates; one 

 between the bases of each two of the arms being larger than the others, while 

 on the anal side, it has numerous small plates with a minute opening, and the 

 whole crowned by a single, much larger plate than any of the others. Several 

 of the species described by Casseday and Lyon, seem to have a similar vault. 



Austin's figurs of D. elongatus, would appear to have the vault terminated 

 by a moderately produced proboscis, with a rounded entire extremity; but as 

 he says some of his specimens (including the one from which the figure alluded 

 to was drawn) have but five pieces in the next range about the base, it is pro- 

 bable the one figured with a proboscis is a true Platycrinus. 



In the simplicity of the structure of its body, this genus resembles Platy- 

 crinus and Hexacrinus, but is readily distinguished from both by its bipartite 

 base. It is more nearly allied, however, to Pterotocrinus, of Lyon, with which 

 it agrees in the number of its basal pieces, and in having the five first radial, 

 and the first anal pieces resting directly upon the base. It differs, however, 

 Widely in the structure of the parts above, particularly in not possessing the 

 greatly developed interbrachial appendages of Pterotocrinus, and in having the 



* Mr. Lyon has suggested that there may be some error in Austin's figures repre- 

 senting four primary radials, in their D. fusiformis, which is certainly very probable, 

 since none of the other speeies are known to have mora than three of these pieces to 

 each ray; and as but one species is supposed to have only two, it is highly probable 

 that the normal number is three. 



