Klem — The Development of Agaricocriaus. 171 



which extend to the arm bases and beyond the second anal. 

 This is not constant, however, the anal being smaller than 

 the radials and of various shapes in many specimens. The 

 next row generally consists of two short plates, succeeded by 

 numerous small, irregular pieces, forming the anal area. 

 This row frequently comprises three, four or five plates, which 

 may be as wide as long, as large as the radials, or almost twice 

 as large. 



Upon examination the interbrachials will be found to be as 

 variable in size as the radials and distichals. In some the 

 first plate rises almost to a level with the arm bases, in others 

 it barely reaches the middle of the second radial. As a re- 

 sult the second brachial plates come much lower, which makes 

 them more prominent in the structure. They are brought in 

 contact with the second radials, cutting off their upper 

 lateral angles, making these plates hexagonal instead of 

 regularly quadrangular. The two of the second row are, as 

 a rule, twice as long as the first and very narrow, while the 

 plates of the third range are quite variable in form and size 

 and often partly interambulacral. The interambulacral areas 

 are filled out with from five to nine small convex pieces. The 

 variations in the proportions of the interbrachial plates can- 

 not be of specific importance because, if the first inter- 

 brachials are small, the second ones are necessarily forced 

 lower down in the cup and become more prominent, while, 

 if the first interbrachials are long, there is no need for the 

 second ones to be large and they remain narrow and do not 

 truncate the second radials on the upper lateral edges as in 

 the former case. 



In their normal state the Agaricocrini of the Chouteau and 

 Burlington Groups have ten arms and those of the Keokuk 

 Group twelve arms, three upon each of the posterior rays 

 and two upon each of the others. The number of arms is 

 not constant however ; additional arms frequently appearing 

 as the result of abnormal development, or one or more arms 

 remaining undeveloped. When a series of specimens, such 

 as I have figured, is carefully examined, the most striking 

 feature is seen to be the irregularity in the number and 

 size of the arms. The normal forms in the Keokuk Group 



