MONOGRAPH OF THE EXISTING CRINOIDS. 181 



Through such forms as Himerometra and Pontiometra we appear to find a 

 complete intergradation between the muscular and the nonmuscular articulations. 



It is noticeable that the highest development of the muscles on the radials 

 occurs on the 5 and 10 armed species, while the last is found in the highly multi- 

 brachiate types, and this might be supposed to indicate that the gradual loss 

 of the muscles was correlated with progressive specialization and therefore that 

 the primitive form was the type in which the muscles are developed to the highest 

 degree. But highly developed muscular articulations giving great flexibility to 

 the arms characterize comatulids with very greatly elongated arms the very 

 numerous brachials of which, particularly the distal, are to the highest degree 

 developed and specialized as uniserial elements of an attenuated appendage, and 

 their appearance is first at or near the arm tips from which they gradually extend 

 themselves toward the basal portion of the arms. In the highly multibrachiate 

 species in which the number of division series is large and the visceral mass 

 extends beyond them to the base of the free undivided arms the plates of the 

 division series become broadened and thin as they are in those fossil forms in 

 which they are incorporated in the body wall, and in their mode of union as 

 in their outer attributes revert to a more primitive type. We appear to be 

 justified, therefore, in considering the type of muscular articulation on the distal 

 end of the radials in Himerometra and Pontiometra as more primitive, by correla- 

 tive reversion, than that on the distal ends of the radials in Pentametrocrinus. 



The more or less complete loss of the muscles in the proximal muscular 

 articulations in multibrachiate comatulids, culminating in their entire absence 

 in Pontiometra andersoni, suggests that the muscles are a purely secondary feature 

 of these unions. The invariable occurrence of a pair of large ligament masses 

 ventral to the fulcral ridge, which gradually decrease in size as the muscles 

 increase, also indicates that the brachial articulations are only secondarily mus- 

 cular, for in a union primarily muscular we should expect to find only muscles 

 on one side of the fulcral ridge and only ligaments on the other, a condition toward 

 which the articulations between the comatulid brachials are evidently tending. 



The numerous successive pairs of muscles between the brachials in the 

 crinoid arms probably represent collectively a single pair of longitudinal muscles 

 comparable with the radial longitudinal muscles of the holothurians or the 

 Echinothuridae. 



Straight muscular articulations. 



The straight muscular articulations have already been described in detail 

 in the section dealing with the articular faces of the radials (pp. 1-77). 



Oblique muscular articulations. 



While in the straight muscular articulations the transverse ridge traverses 

 the joint face at right angles to its dorsoventral plane, the two interarticular 

 ligament and muscular fossae being similar and of equal size, and viewed dorsally 

 (exteriorly) the joint face is seen to cross the arm at right angles to the longi- 

 tudinal axis, in the oblique muscular articulation (figs. 1083, pi. 18; 1110, pi. 20; 



