218 PALAEONTOLOGY OP ILLINOIS. 



From these figures and his description, with what is now known 

 of several American species of this type, it is evident that the 

 formula, in accordance with the later improved nomenclature, 

 may be stated as follows : 



Generic formula. — 



Basal pieces, 5. 



Subradials, 5. 



Radials, 3+5. 



Secondary or supraradials, 3 or 4 + 10. 



Anal and interradial pieces, 12 to 15 + 5. 



Pseudo-brachial appendages (arms of some authors), 5, or rarely 6 (from 



the division of one on the anal side) ; located over or between the rays. 

 Arm-openings (ambulacral) 10, located directly under the pseudo-brachial 

 appendages, or alternating with them. 



The genus may therefore be characterized thus : body usu- 

 ally short subcylindrical. Base small, formed of five penta- 

 gonal pieces, composing a more or less impressed pentagonal 

 disc. Subradials five, alternating with the basal pieces, all 

 hexagonal or obscurely heptagonal, and truncated above for 

 the reception of the first anal and first interradial pieces; 

 like the first radials, often protuberant or extended in the 

 form of spines. Primary radials in five series, of three each, 

 and all alternating with the subradials; the first one of each 

 series heptagonal, the second hexagonal and the third hexago- 

 nal or heptagonal, and supporting on its superior sloping sides 

 the diverging secondary radials. Of the latter, there are two 

 or three in each range, the last one of which snpports on its 

 inner sloping sides the delicate pendent arms; while in one 

 section of the genus, the outer sloping side of each of these 

 pieces bears another piece, which, owing to the divergence of 

 the secondary radials, meets the corresponding piece of each 

 adjacent ray (excepting rarely on the anal side) over theinter- 

 radial and anal spaces, and there forms the base of the»stout 

 false arms, which, in the typical species of the group, stand 

 over the interradial and anal spaces. Interradial and anal 

 areas oval, often scarcely distinguishable from each other, and 



