19g proceedings of the academy of [1881. 



6, Arms and 1'innules. 

 The arms in the Sphseroidocrinidae are either simple or branch- 

 ing, and are constructed either of a single or double series of 

 joints. Single arm joints are restricted to the Silurian, all 

 Devonian and Subcarboniferous genera have two rows of alter- 

 nately arranged joints in the arms. In the Cyathocrinidfe, almost 

 to the close of the Subcarboniferous, the arms are composed of 

 single joints, but in the Kaskaskia Limestone and Coal Measures 

 a few species occur with double-jointed arms, intermingled, how- 

 ever, Avith species apparentl}' of the same genus, in which the arms 

 are constructed of single wedge-form joints alternately arranged. 

 These two structures run so closely into each other through 

 transition forms, that we have been compelled to arrange them 

 in some cases under the same genus, although we have in. 

 other cases considered the arm structure to be of generic import- 

 ance. For tliis we have been criticised by Prof. Wetherby, of 

 Cincinnati, who thinks it " a singular statement, that a character 

 in forms of the same geological age may be generic in one case, 

 and only specific in another." He evidently overlooks the fact 

 that all Crinoids in their young stage have single-jointed arms, 

 and that the double-jointed feature is a product of growth which 

 primarilj- was only an individual variation, but which, by becom- 

 ing fixed and constant in certain forms, attained generic value, 

 especially when taken in connection with other characters. The 

 best specific and generic and even famil}' characters, originated in 

 individual A^ariation, and at some period in the paleontological 

 history of the organism were without value as a means of classi- 

 fication. Near the close of the existence of the family Cyatho- 

 crinida?, the double-jointed arm structure began to be developed, 

 in some forms irregularly, in others to such a degree as to be 

 constant, and to form a distinguishing characteristic of many 

 species, which thus fell naturally into a generic group. This 

 process is illustrated in Eupachycrinus, Hydreionocrinus and 

 Erisocrinus^ and there are similar examples among the Sphseroido- 

 crinidae. The young Platycrinus and Actinocrinus have a single 

 series of cuneiform arm joints, which are constructed exactly like 

 those of the adult Poteriocrinus^ and the pinnules are given off in 

 a like manner. At a more advanced stage the joints begin to 

 enlarge laterally in such a manner that the sharp inner angles 



