116 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



As will be explained later, each pinnule is in origin an atrophied arm, and 

 arm branching of this type merely indicates that a pinnule has more or less 

 completely reverted to its original intent. 



NomencJature of the different types of post-radial structure. 



When the postradial series are undivided and the first brachial pair imme- 

 diately follows the radials. as in the Pentametrocrinidse and in Atopocrinus, the 

 arms are said to possess Simple Arm Structure. 



All the 10-armed species, except those included in the genus Thaumatocrinus, 

 together with the species of Promachocrinus and Eiidiocrinus, in which the first 

 brachial pair at the base of the free undivided arm is preceded by a single 

 reduplication of itself (the IBr series), are said to have Simple Interpolated Arm 

 Division. 



Species with more than 10 arms and with all the division series 2 or 4(3-1-4) 

 and the first brachial pair present at the base of the free undivided arms are 

 said to have Multiple Interpolated Arm Division. 



This multiple interpolated arm division may be Primary, where all the 

 di\-ision series are 2, or it may be Secondary, where more or fewer of the division 

 series are 4(3-|-4). 



When the first element of the first brachial pairs and their reduplications 

 is absent from all the pairs beyond the IIBr axillary, as in CapiUaster , and 

 Nemaster, the postradial series are said to possess Defective Interpolated Arm 

 Division. 



When the external arms on each axillary arise through multiple interpolated 

 division, but the internal arms lack the first brachial pairs, as in CornateUa, 

 iVeocomateUa, and Palceocoviatella, the postradial series are said to have Compound 

 Interpolated Arm Division. 



Metacrlnus. in which the first brachial pair is represented by the first two 

 ossicles following the radials, but in which the arm divides several times distal 

 to it at irregular intervals, is said to possess Simple Extraneous Arm Division. 



Such genera as Isocrinus and Pentacrinus {Extracrimis), in which the first 

 or first two division series are interpolated, but all those following extraneous, are 

 said to possess Heterotypic Arm Division. 



ADOLESCENT AL'TOTOMT AND THE REDUPLICATION OF THE ARMS. 



Yot(n<7 of the Comatulids. 



In the young of the species of Eudiocrinus and of the Pentametrocrinidse the 

 arms from their first inception grow continuously and uninterruptedly during 

 the life of the individual, though in the adults the addition of new brachials, 

 which takes place at the arm tips, proceeds very slowly. 



Likewise, in the 10-armed species the arms, forking soon after their first 

 appearance, continue to grow uninterruptedly throughout the life of the individual. 



In all the comatulids the arms almost from the very first exhibit all the 

 essential characters of the arms of the adults, and the only change that takes 



