MONOGRAPH OF THE EXISTING CRINOIDS. 183 



pinnule socket beinp; in the right muscular fossa, the transverse ridge of the next 

 succeeding (or next preceding) oblique muscular articulation -will be found 

 always to cross the joint face from a left dorsolateral to a right ventrolateral posi- 

 tion, and the pinnule socket will be found always in the left muscular fossa. The 

 fulcral ridges of succeeding oblique muscular articulations make an angle with each 

 other typically of approximately 90°. This alternation in the direction of the ful- 

 cral ridges accounts for the marvelous flexibility of the crinoid arm. 



The straight muscular articulation admits of motion only in the dorsoventral 

 plane. The alternation in the direction of the fulcral ridges of the oblique mus- 

 cular articulations admits of motion, coUectivelj^ in any direction; but the arm 

 can not be rotated. Synarthries admit of motion in the plane at right angles 

 to the dorsoventral plane, but the scope of synarthrial motion is always very 

 limited. Syzygies admit of no motion whatever. 



While the straight muscular articulation is invariably single, oblique mus- 

 cular articulations are always double. Except in Eudiocrinus, Atopocrinus, and 

 the Pentametrocrinidse, the first oblique muscular articulation always occurs in 

 the form of two similar articulations situated side by side on the distal end of 

 the second postradial ossicle (fig. 1102. pi. 19), and several of those immediately 

 following may be developed in the same way. The first oblique muscular articu- 

 lation of the free undivided arm consists of the articulation uniting the second 

 with the third brachial, plus a large and well-developed pinnule socket, and the 

 following articulations are similar except that the pinnule socket rapidly diminishes 

 in size and perfection. 



In the recent comatulids the two articular faces on the distal end of each 

 axillary are always exactly alike, but in the genus Metacrinus the axillaries beyond 

 the first always have one of the two articular faces and the arms arising therefrom 

 larger than the other, and the difference is sometimes very marked. In the closely 

 related genus Pentacrinus (fossil only) the discrepancy in the size of the two faces 

 becomes much more marked, so that in each postradial series there are commonly 

 four main arm trunks gi^^ng off ramules at fairly regular intervals. 



The condition normal in Metacrinus and Pentacrinus occurs sporadically in 

 the comatulids, and it is not unusual, especially in Tropiometra picta (figs. 173, 

 p. 89, and 1027, pi. 11), to find a .specimen in which one of the distal pinnules has 

 been replaced by a more or less perfectly developed arm. Replacement of a pinnule 

 by an arm is much more common, however, on the second postaxillary ossicle than 

 distally. In Coviatula etheridgei (part 1, fig. 78, p. 131) this ossicle normally bears 

 arms which are only a fraction of the length of the normal inner arms, but in most 

 cases the supplementary arm is of full size. 



It occasionally happens that an axillary bears two pinnules instead of two 

 arms, replacement of an arm by a pinnule occurring as well as the converse. 



Since axillaries bear arms of all degrees of diversity in size from two exactly 

 similar arms to an arm and an insignificant ramule, and any pinnule may be re- 

 placed by an arm or any arm by a pinnule, it seems logical to assume that pinnules, 

 interchangeable with arms, are but dwarfed arms themschcs derived through 

 142140— 21— Bull. 82 14 



