202 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The first syzygial pair is composed of the third and fourth brachials, the 

 second of the ninth and tenth, and the third of the fourteenth and fifteenth, more 

 rarely of the seventeenth and eighteenth. Beyond this point syzygial pairs occur 

 regularly separated by one, two, three, or four, most commonly two or three, single 

 brachials, the number being more or less directly correlated with the size of the 

 species. 



The long interval between the first and second syzygial pairs is a heritage from 

 the ancestral Flexibilia arm, and is correlated with the retarded development of the 

 pinnules borne by the intermediate brachials. 



In the secondary arms of the multibrachiate species the brachials become more 

 uniform and more wedge-shaped, with less oblique ends, the pinnules become more 

 uniform and stouter, the syzygial pairs become much less frequent and more widely 

 spaced, and the articulations between the pinnulars take on more and more the 

 character of true muscular articulations. 



In the Atelecrinidse and Pentametrocrinidse, in which the arms are unusually 

 long and very highly developed, the syzygial pairs are more widely spaced than in 

 the arms of ordinary 10-armed types, and, excepting only in Atelecrinus, the first 

 syzygial pair is separated from the distal element of the first brachial pair by an 

 interpolated ossicle. 



In the distal part of the arms as the brachials become more and more per- 

 fected in the direction of a linear series of ossicles the two elements of the syzygial 

 pairs become less and less individualized, each pair taking on more and more the 

 characteristics of a single normal brachial. 



Radials. The radials have always been considered as true calyx plates, and the 

 radial circlet has, therefore, been assumed to be quite as much an integral part of the 

 crinoid calyx as the circlet of basals or of infrabasals. 



The radials have the following characteristics of true calyx plates : 



In very many types they form, in the shape of a broad closed ring resembling 

 the basal ring, an integral part of the cup in which the visceral mass is lodged. 



Alternating with the basals, they are usually interlocked with them in the same 

 way that they are interlocked with the infrabasals. 



The size of the radials often bears roughly the same proportion to the size of 

 the basals as the size of the latter does to that of the infrabasals. 



The arms often arise as slender free appendages from the middle of the distal 

 border of the radials, while the latter are incorporated in the wall of the capsule 

 inclosing the visceral mass. 



That these characteristics are not fundamental attributes of the radials is evi- 

 dent from the following facts : 



Very frequently the radials do not form a closed ring, but are separated from 

 each other by interradials, which are particularly likely to occur in the posterior 

 interradius. They may become, as in the comatulids and pentacrinites, so altered as 

 to cease to function as an integral part of the body wall, serving merely as bases 

 upon which the arms are developed ; furthermore, a variable number, often large, 

 of the postradial plates may become incorporated in the body wall. 



