INVERTEBRATES. 101 



of the rays, while the sixth crowns the summit of the dome. 

 These spines are sometimes bifurcated as seen in the example 

 figured. 



Bawal plates massive, spreading at their lower margins and 

 separated by clearly defined sutures. First radial plates hexa- 

 gonal, about twice as wide as long. The left antero-lateral ray 

 in the specimen figured has but two radials, the second of which 

 is an axillary plate, supporting on its upper sides two secondary 

 radials. In the other rays there are three radials, the second 

 of which is quandrangular in outline, the third pentagonal, 

 and gives support to two pentagonal secondary radials. These 

 are succeeded by four tertiary radials that support the arms, 

 the normal number of which is four to each ray. The plates 

 forming the dome are more convex than those below the arms. 



Interradial plates three visible below the arms, the first large, 

 and heptagonal, the other two smaller and hexagonal. First 

 anal plate heptagonal, length and breadth nearly equal, and 

 supporting three smaller anals in the second series. 



Column round and consisting near the summit of alternating 

 thicker and thinner joints, the latter increasing in number as 

 the distance from the body increases. 



Position and locality: Keokuk limestone, Hamilton, Illinois. 



This magnificent specimen was collected by Mr. L. A. Cox, 

 by whom it was presented to the author. 



Collection of A. H. Worthen. 



