INVERTEBRATES. 845 



The interradials of the four lateral rays, toward the anterior side, 

 consist of seven plates in four series, 2, 2, 2, 1 ; at the anterior side 

 of only six pieces, 2, 1, 2, 1. The plates of the first series rest 

 with their lower sloping sides against two of the intercalated pieces 

 those composing the second ring around the basals ; they are placed 

 with their lateral side against the sides of their fellows of the ad- 

 joining interradial area, -except toward the anterior and posterior 

 side, where they abut, at the former against the first radial, at the 

 latter against one of the special anal plates. The plates of the 

 succeeding series rest with their outer sides within the angle be- 

 tween two radials, with the other side against the adjoining inter- 

 radials. The plates of the interradial areas are either hexagonal or 

 pentagonal, and their increase in size in an upward direction is 

 even greater in that same direction among the radials, the upper 

 one being fully three times larger than the two interradials of the 

 first series. The same is true with regard to the azygous side. 



The anal side is very wide, and is composed of a median row of 

 four hexagonal pieces, longitudinally arranged, and of a series of 

 seven plates at each side of it. The latter are arranged like the 

 seven plates of the two interradial areas adjoining the anterior ray, 

 and as the radials of that side, as already stated, are arranged and 

 constructed like the special anal plates, the anterior side, with its 

 two interradial series, is almost a perfect counterpart of the anal 

 area. The uppermost anal plate is hexagonal or, perhaps, octagonal, 

 its upper side truncate ; its size is equal to, if not larger than, that 

 of the first radials. 



Acrocrinus Wortheni differs from the two previously described species 

 of this genus most conspicuously in the form of the basal plates. 

 The construction of the other plates is so imperfectly known that a 

 comparison of other parts is impossible. There is also a great dif- 

 ference in the form of the body. 



The specific name is given in honor of Prof. A. H. Worthen, the 

 able director of the Illinois geological survey. 



Geological position and locality: From the Coal Measures of Peoria 

 county, 111. 



The type is in the Illinois State collection. 



