524 GEOLOGICAL SURVEY OF IOWA. 



FOSSILS OF THE BURLINGTON LIMESTONE. 



CRINOIDKE. 



IN the following descriptions of the Crinoideae, I have 

 adopted with slight modifications the terminology proposed 

 by DE KONINCK and LE HON in their " Recherches sur les Cri- 

 noides du Terrain carbonifere de la Belgique"; this being 

 better adapted to a strict designation of the parts, than the 

 momenclature of MILLER, or any modifications of that which 

 have been proposed. Some latitude is necessary in the gene- 

 ric descriptions, unless we would multiply genera to an 

 unnecessary extent. 



The species of the Genus ACTINOCRINUS assume such a va- 

 riety of forms, with such modification of the rays, that it 

 becomes necessary to extend the terms designating the parts. 

 We have among the Actinocrinides, those where the arm-plates 

 rest directly upon the last or third radial plate ; and we have 

 others where but a single plate intervenes ; while beyond 

 this are various modifications involving a subdivision of the 

 series beyond the third radial plate, with one, two, three, 

 or more plates intervening between these, and those which 

 can be strictly denominated arm-plates. As these interme- 

 diate plates form an essential part of the body, and, as I 

 believe, belong to the true radial series, I have designated 

 them supraradial plates. The supraradial series is sometimes 

 again subdivided on one or both branches of the ray, as in 

 those designated by Drs. OWEN and SHUMARD Megistocrinus, 

 as well as in some other forms which are so clearly the type 

 of ACTINOCRINUS proper, that no separation would be pro- 

 posed. In these forms the interradial and anal plates are 

 greatly increased in number, beyond those forms where the 

 radial plates are succeeded directly by the arm-plates. There 



