170 Trans. Acad. Sci. of St. Louis. 



The first radials vary much in size, being the largest plates 

 of the dorsal cup in some, and quite small in others. In 

 many they are irregular in size, the posterior ones being 

 generally longer than the others. Often the sides are spread- 

 ing, the lower ends thickened to form a circular ridge around 

 the depression for the column. The second and third 

 radials are among the most variable plates in the disk. 

 The second radials are usually quadrangular with convex 

 sides, but frequently one or both upper angles are trun- 

 cated by the second interbrachials. In some three are 

 hexaugular, and the two posterior ones pentangular; or all 

 are quadrangular, except the one on the anal side which has 

 one of the upper angles truncated, making a fifth angle. 

 There is considerable range in size ; in some specimens they 

 are larger than the first radials ; in some they are the largest 

 plates in the disk, and in others they are very short, more 

 than twice as wide as long. The third radials are regularly 

 five-sided, but pieces with six, seven and eight sides are of 

 common occurrence. Sometimes they are irregular in out- 

 line, or all pentangular, except the posterior ones which are 

 hexangular. In some they are triangular and so small that 

 the first piece of the brachials rests in part on the second 

 radial as well as on the third. As the first and second radials, 

 so these may be the largest plates in the disk. Often they 

 are tumid and project beyond the surface of the first and 

 second radials. 



The distichals and brachials are quite variable in form and 

 size, as well as in number among the rays. In many the 

 first distichal of the three anterior rays is followed by a 

 cuneate second plate, while the posterior rays have one 

 distichal, followed by two palmars, or only one distichal with 

 one palmar. The second distichal may be quite small and 

 cuneate, the outer ends being occupied by arm plates which 

 meet it from opposite sides. Usually there are two series of 

 alternating brachial pieces which form the base of two arms. 

 Additional arms are produced by the intercalation of brachial 

 pieces between the others. 



The first anal is usually longer than the radials and fol- 

 lowed bv a second anal and an interbrachial on each side, 



