PALAEONTOLOGY OF IOWA. 543 



ZEACRI KITES ( TROOST, Catalogue of Crinoidea). 



Among the Crinoidese of the Carboniferous limestone there 

 is a group of species which possess the general characteris- 

 tics of POTERIOCRINUS, but which have a less robust habit. 

 The base is always rotund, more or less depressed, and often 

 deeply concave in the centre : the basal plates are usually 

 hidden by the column, while there is not unfrequently some 

 irregularity in the subradial plates. The arms are usually 

 closed. The general form of body and arms is terete or sub- 

 cylindrical, often contracted at the base of the arms, and 

 sometimes spreading above from the frequent branching. 



In several specimens examined there is an irregularity in 

 the anterior ray, and a greater number of plates by one or 

 two or more below the first bifurcation, than in the other 

 rays. In all the species of this character the mode of arti- 

 culation of the plates agrees, and differs from the true PO- 

 TERIOCRINUS; and we find at the same time, that the radial 

 series always presents two plates in succession below the 

 first bifurcation of the arms : and it might, with propriety, 

 be described as having radials 2x5, while the mode of ar- 

 ticulation of the radial and arm-plates is so different from 

 the typical forms of that genus that it furnishes a charac- 

 teristic distinction. 



The line of articulation between the radial plates is slightly 

 concave or nearly rectangular to the axis ; and while the 

 edges are in contact along the centre, there is usually a hiatus 

 between the plates on the outer edge, as if the space had been 

 filled with cartilage, which has decayed, leaving the present 

 unoccupied space. 



For a well marked species of this type, the late Dr. TROOST 

 proposed the name of ZEACRINITES, and the species occurs in 

 his Catalogue as Zeacrinites magnolittformis. No generic de- 

 scription accompanies the description of the species ; but 



