PALAEONTOLOGY OF IOWA. 525 



are, likewise, in the species illustrated, great variations in 

 the development of the basal plates, which, from being large, 

 and projecting over the column in a broad rim, are in other 

 species so little developed as to be almost entirely hidden 

 by the summit of the column. 



In its fundamental structure, the Genus AGARICOCRINUS of 

 TROOST does not differ essentially from Actinocrinus ; the 

 chief variation being in the interradial and anal plates, while 

 all the species known have extremely small basal plates, 

 scarcely reaching beyond the column. In all species the 

 brachial plates proceed from the third radial, and the arms 

 are always in pairs from each ray. 



Variations of type and modifications of generic form will 

 be found to mark the species of the successive limestone 

 formations, each one of which is well characterized by the 

 remains of very distinct species of this family of fossils. 



Among the species described in the Keokuk and Chester 

 limestones, it gives me great pleasure to be able to recog- 

 nize several which are published in Dr. TROOST'S Catalogue, 

 but of which no descriptions have heretofore been given. I 

 have, in these cases, adopted without hesitation the name 

 given by that author. 



GENUS PLATYCRINUS (MILLER, 1821). 



" Pelvis composed of three unequal plates, supporting five 

 " scapula." 



FIG. 55. GENERIC FORMULA, according to 



DE KONINCK. 

 Basal plates, 3. 



Radial plates, 2X5, the first series large and the 

 second small. 



Anal plates, 1 large and 3 small. 

 Interradial plates, 1X4. 



Number of arms, 10, 20, 25, 30, 35 [and 40, 45], 

 according to the species. 



