UINTACRINUS: ITS STRUCTURE AXD RELATIO^'S. 



85 



Inspection of the foregoing table shows that there is no normal number 

 of interbrachials, and that no regularity can be found in the variations 

 except such as are traceable to individual growth. The greatest regularity 

 is in the smallest specimens, which have as a rule five or six plates to the 

 area, and frequently the same number in all tlie areas exposed. As the 

 size increases, the number of interbrachials increases in one or more inter- 

 radii, but scarcely ever the same in all. From the nature of these plates 

 we should expect to find just this kind of irregularity. For it is not prob- 

 able that the plates of the calyx and their ligamentous or muscular con- 

 nections would be of equal strength throughout, and hence the expansion 

 of the visceral mass would cause the plates to open, or new stereora to be 

 deposited, first at the points of least resistance. Therefore, while the num- 

 ber of interbrachials might be quite uniform in young individuals, we can 

 readily see how one or more supplementary plates might be introduced in 

 one interradius and not in another. Among the adult specimens numbers 

 from eight to ten, in which there is generally a central plate surrounded 

 by a band of others, are the most frequent ; but 23 per cent of the speci- 

 mens have their iBr spaces composed of seven plates or less, in which there 



