168 



Trans. Acad. Sci. of St. Louis. 



plates are always large and prominent in this genus ; the 

 posterior one being usually larger than the other five and 

 central. They divide the ventral side into five spaces occupied 

 by smaller plates. Four of these areas are about the same 

 size, while the one on the anal side is much larger. (See 

 figure.) The condition of the orals varies considerably; 

 sometimes they are tuberculose as in A. excavatus, nodose as 

 in A. bullatus, spinous as in A. stellatus, or conical as in 

 A. Coreyi. 



DEVELOPMENT OF AGARICOCRINUS AMERICANUS.* 



The next step in the development was the formation of a 

 series of smaller pieces around each oral. This ring can be 

 traced more or less distinctly in every instance, but in no two 

 cases is it composed of the same number of pieces, or of pieces 

 of the same size. The variations in the size, shape and number 

 of these plates, in all probability, depended upon the quality 

 and the quantity of food, the amount of light and the nature 

 of the surroundings. If the animal was healthy, well-fed, and 

 the environments were favorable for rapid growth, it seems 

 plausible to suppose that it developed large plates, but few of 

 them ; if the nutrition was poor and the conditions were 

 adverse, then a large number of much smaller plates formed. 

 Very often we find a number of small plates inserted between 



* The six plates shaded heavily were the first plates to be deposited. 

 The series of smaller plates around each, which are shaded lightly, were 

 the second step in the development. The intervening pieces, which are 

 not shaded, were formed at a later period as the necessary material 

 could be produced. 



