PALEONTOLOGY OF IOWA. 559 



GENUS SYNBATHOCRINUS (PHILLIPS). 



FIG, 76. 



GENERIC FORMULA. 



Basal plates 3, usually united as one piece. 



Radials 2 X 5 =; 10. 



Anal plates 1, 2, or more. 



Interradials plates 0. 



Arras simple ( double ? ) , and composed of plates in 



i 



THE species of this genus are distinguished by a generally 

 elongated, cylindrical, or subfusiform character. The base is 

 usually small, consisting of three plates which are often so 

 closely united as to appear as a single piece. Of the first 

 radials, two are quadrangular, two pentagonal, and the fifth 

 is sometimes slightly truncated on one of its upper angles 

 by the anal plate. Second radials quadrangular. Arms simple 

 (or double?) : plates with the articulating surfaces parallel, 

 very gradually tapering. 



Prof. PHILLIPS has described this genus as having a solid 

 base. Prof. TROOST, in his Memoir on the Crinoideae, has 

 described the base as composed of three pieces in the young 

 state ; and Dr. SHUMARD has made the same observation. In 

 the specimens before us, we have evidence confirming the 

 truth of these observations. 



In general form, the species of this genus resembles 

 GRAPHIOCRINUS ; but the solid base, or three basal plates con- 

 stitute a distinguishing feature. 



Niagara group, in its rotund base, impressed column area, absence of basal plates, much 

 shorter arms, and general globose form of the whole body. 



Dr. TROOST does not decide the position of his species, but remarks that it was found 

 among the disintegrated materials of Carboniferous and Devonian strata at White's Creek 

 Springs. Many of the species of ACTINOCRINUS. and others from this locality, are from 

 the horizon of the Keokuk limestone ; and from the absence of the Burlington limestone 

 in the southwest, we may presume that I. tiareeformis is of the age of that formation. 



