no. 1636. ARM HOMOLOGIES IN RECENT CRINOIDS— CLARK. 



127 



of Isocrinus all the synarthries are replaced by syzygies, and all the 

 divisions are interpolated, consisting of two joints. Isocrinus decorus 

 and /. blakei, in the ten-armed immature state, are exactly similar in 

 arm structure to Isocrinus naresianus and the ten-armed comatulids, 

 excepting - Uintacrinus and Decametrocrinus. In the adult multi- 

 brachiate condition, however, instead of adding interpolated joint 

 pairs as in the comatulids and in the species of Isocrinus just con- 

 sidered (parrce, wyville-thomsoni, alternicirrus, and sibogce) , the 

 arm branching, as in /. listeria (fig. 27), is of the extraneous type, as 

 in Comaster (as restricted), Z x and Z 2 

 remaining always the first and second 

 joints after the first axillary, or the third 

 and fourth after the radial, as was 

 found to be the case in Comaster; and, as 

 in Comaster* the syzygy between the two 

 joints following Z 2 is morphologically the 

 syzygv between the third and fourth joints 

 of the undivided arm in the ten-armed 

 young, and comparable to the similarly 

 situated syzygy in all ten-armed comatu- 

 lids, while in Phanogenia and other forms 

 in which the second division series is of 

 four joints, the two outer united by syzygy, 

 the syzygy is morphologically homologous 

 with the synarthry between the first two 

 joints in the free undivided arm, and all 

 other syzygies and synarthries proximal to 

 it. In other words, the syzygy between the 

 third and fourth joints after the first 

 axillary in Comaster, Isocrinus blakei, I. 

 decorus, and /. asteria, is homologous with 

 the first syzygy in the free undivided arm 

 in all other forms (except in cases where 

 the first syzygy replaces a synarthry) and 

 with no other, no matter how many syzy- 

 gies may intervene between that syzygy 

 and the radials. 



An extraneous division, arising as it does from a division of the arm 

 at an oblique muscular articulation, might reasonably be supposed to 

 be of somewhat uncertain nature in the position of the succeeding 

 axillaries, because of the fact that all the arm joints after Z 2 , except 

 occasional syzygies, are thus articulated, and, of course, every such 

 articulation is a potential axillary; and, as a matter of fact, this is 

 the case; while in the type of Comaster considered the division was 

 regular in the number of joints between successive axillaries, in 



Fig. 28. — Pentacri.\itid.£ ; 

 Metacrinus. 



