92 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 41. 



spoil the force of the argument. "Among the 268 dicycHc speci- 

 mens examined by me / can scarcehj point out one in which the coal- 

 escence of the infrabasals and centrale would produce a plate having 

 the same outhne as the centrale has, or which would not be entirely 

 distinct in shape from the centrale in most of the 275 monocyclic 

 specimens in the collection." These irregularities in the shape of 

 the centrale will be discussed later. At this point let me merely 

 point out that in the case of Carahocrinus already cited we have a 

 stellate opening to be filled, but the centrale which is formed is 

 strictly pentagonal and, moreover, interradial. 



It is a notable fact, although one which I believe has never been 

 hitherto pointed out, that a permanent reentrant angle is an un- 

 known quantity in a fully formed crinoid plate. This rule applies 

 with equal force to a composite plate resulting from the complete 

 fusion of two or more primary plates. Indeed, I hold that this rule 

 is equally applicable to all Echinoderms. The forked radial of the 

 Blastoidea wiU suggest itself to everyone as an exception to the 

 rule. Even in this case, however, the exception may prove more 

 apparent than real. 



In the case of fusion among basals and infrabasals in crinoids the 

 resultant reentrant angle formed between any two of the plates 

 tends to fill out and make a straight face. At the same time there 

 is a gradual resorption of the lower edge of the plate resting in the 

 reentrant angle. Within a short time all traces of the reentrant 

 angle disappear and a straight face results. Why this is so it is 

 hard to say, unless it be that a reentrant angle is a source of weak- 

 ness in a plate and is eliminated as rapidly as possible. A large 

 number of examples of such filling up of reentrant angles might be 

 adduced. An examination of a series of Camerata in wliich the 

 basals tend to fuse into a solid plate will illustrate this condition 

 admirably. In certain species where fusion has not become effec- 

 tive, or in young individuals, it is to be noted that certam of the 

 radials, and frequently the proximal anal plate, if such be present, 

 rest in reentrant angles formed by the basals. Upon a complete 

 consolidation of the basals a pentagonal or hexagonal plate with 

 straight faces is formed. Such a hexagonal disk as is shown in the 

 genus Megistocrinus is here figured (PI. 9, fig. 12). In other speci- 

 mens of Megistocrinus it may frequently be observed that the anal 

 rests within an angle formed by two of the unfused elements of the 

 tripartite base. 



The regidar polygonal shape of such basal elements as is to be 

 noted in the Camerata may only be accounted for on the basis of 

 fusion and subsequent filling of reentrant angles. Wliere before 

 there was a reentrant angle between two plates, after such fusion 

 we find a straight face. This change necessarily requires not only 



