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



119 



Fig. 16. — Ztgomet- 

 uid.e ; Catopto- 



METBA. 



Atelecrinidce (fig. 18) and Antedonidm" (fig. 17). — In these fam- 

 ilies the arms, instead of remaining single throughout, fork at the 

 second post-radial joint; this is a matter of no real importance so 

 far as the arm structure goes, for it must be remembered that any 

 muscular articulation, whether straight or oblique, occurring at the 

 distal end of a joint may divide and form hvo, 

 from which two similar arms arise; the important 

 thing is not the forking of the arms, but the de- 

 termination to what type of muscular articulation 

 belong the articular faces on the distal end of the 

 axillary. 



Bearing this in mind it will be found that the 

 sequence of articulations of these two families is as 

 follows: straight muscular between the radials and first post-radial 

 joints; synarthrial between the first two post-radial joints; .straight 

 muscular between the second post-radial (axillary) and third post- 

 radial (first post- axillary) joints ; synarthrial between 

 the third post-radial (first post-axillary) and fourth 

 (second post-axillary) joints, and oblique muscular 

 between the fourth and fifth post-radial (second and 

 third post-axillary) joints. The first and second 

 post-axillary, or third and fourth post -radial joints. 

 therefore, are our Z a and Z 2 , while the first and second 

 post-radial joints (the second an axillary) corre- 

 spond to the first and second post-radial joints in 

 Eudiocrinus (in which the second is not an axillary, bearing merely 

 a pinnule instead of an additional arm), and are really an inter- 

 polated reduplication of the first and second post-axillary joints inter- 

 polated between them and the radials. Now in the 

 Atolecrinida? and Antedonidse, and in ten-armed 

 species belonging to genera in other families (which 

 are constructed upqn the same plan as the universally 

 ten-armed genera and species of Atelecrinidse, and the 

 primarily such of Antedonidse) we are so fortunate 

 as to find additional proof of the correctness of this 

 analysis of the proximal arm structure. In certain 

 species, such as Perometra diomedece, enormous tuber- 

 cles are developed at the synarthry between Z t and Z 2 ; 

 these are always repeated on the synarthry between the first and sec- 



Adelometra angustiradia and occasionally specimens of Antedon bifida 

 have more than ten arms, their structure being then similar to that of the 

 Himerometridse, and multibrachiate comatulids in general, except Comaster 

 (see below) : Antedon ( restricted) . considered by Doctor Carpenter as a primi- 

 tive type, is in reality one of the most specialized genera in the family, ap- 

 proaching the Ilimerometridit' in many ways. 



Antedo- 

 Helio- 



P I g. 18. — A i i:i.i:- 

 ckinid.e ; Ate- 

 lecrinus. 



