312 CLARK : PLATES IN CRINOIDS 



ture of the crinoid arm, exactly as every process from the centro- 

 dorsal will become a cirrus and every one arising on the distal 

 corner of a brachial will become a pinnule. 



Thus while a plate if situated below the ventral edge of the peri- 

 somic surface may give rise to a simple series of more or less simi- 

 lar plates running up to the edge of the ventral surface, and possi- 

 bly continued further along the anal tube, the same plate if 

 situated just at the ventral surface will give rise to an arm or a 

 group of arms exactly like those arising from the radials. The 

 character of the ossicles following a plate is not determined so 

 much by the character of the plate itself as by its position in 

 reference to the boundary between the dorsal and ventral surfaces 

 of the animal. 



I have examined pentacrinoids in which both the radianal and 

 anal x are present, the former dwindling, the latter increasing 

 in size. They are situated side by side between the two posterior 

 radials. 



In some thirty six-rayed specimens which I have studied the 

 supernumerary ray is in all cases but two inserted behind the left 

 posterior; that is, between the two posterior radials, and receiving 

 its ambulacra from the groove trunk to the left. 



It is thus clear that in Thaumatocrinus renovatus we have a 

 young comatulid just after the resorption of the radianal with 

 anal x fully developed and bearing a rudimentary arm which 

 eventually will increase in size and become indistinguishable 

 from the other arms. 



This establishes the identity of the supposed species. Since 

 anal x has given rise to an additional arm it is only reasonable to 

 suppose that all the other interradial plates, which are exactly 

 like it, will do the same thing. There will therefore result a ten- 

 rayed form with ten undivided arms from which IBr series will 

 be absent. Such a condition characterizes the genus Decame- 

 trocrinus, one species of which, D. abyssorum, was the only other 

 crinoid dredged with Thaumatocrinus renovatus, and Decame- 

 trocrinus and Thaumatocrinus have since been found similarly 

 associated. 



