118 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXV. 



Fig. 12. — Pentametrocri- 

 nidje; Thaumatocri 

 nus (adapted from p 

 II. Carpenter). 



simplest type of arm structure possible, Z x and Z 2 , followed by 

 brachials of the type common to the distal 

 part of the arm in all the other types. 



Eudiocrinus (restricted) (fig. 14). — In this 

 genus, in which the five arms are undivided, 

 the sequence of articulations is as follows: 

 straight muscular between the radials and 

 next following joints; syzygy between the first 

 two post-radial joints (therefore occupying 

 the position of a synarthry) ; straight muscu- 

 lar again, a pinnule being developed on the 

 proximally adjacent joints; synarthry (with, 

 of course, no pinnule) ; oblique muscular* a 

 pinnule being developed on the proximally ad- 

 jacent joint on the opposite side to the first 

 pinnule, as pinnules always alternate in posi- 

 tion at succeeding articulations, unless the 

 articulation is a primarily nonpinnulate syn- 

 arthry or syzygy, which has no effect on pin- 

 nulation. In the third and fourth post-radial 

 joints we can again immediately recognize 

 our Z x and Z 2 ; therefore, the first two post- 

 radial joints in the Pentametroerinida? are homo- 

 logous with the third and fourth post-radial joints 

 in Eudiocrinus. But what are the two joints be- 

 tween the radials and Z t ? It is evident that the 

 first post-radial joint agrees with Z 1 in the manner 

 of its proximal and distal articulations, except that 

 the normally present synarthry is replaced by a 

 syzygy, which, as it is morphologically the same 

 thing, is a point of interest, but not of importance; 

 Z 2 agrees with the second post-radial joint in 

 having proximally a synarthrial articulation, dis- 

 tally a muscular (but straight instead of oblique 

 muscular) ; thus we find that the arms of Eudio- 

 crinus resemble those of the Pentametrocrinida\ 

 except that X 1 and Z 2 are repeated, the addi- 

 tional pair being interpolated between Z t and the 



Fig. 13. — Pextametrocri- 

 N I d je : Decametrocri- 



NUS (ADAPTED FROM P. 



H. Carpenter). 



Fig. 14. — Zygomet 

 ridje ; Eudiocrinus 



Fig. 15. — Zygomet- 

 rid.e ; Zygometiia. 



/■(/dials." 



In oases like this where Zi and Z 2 are repeated, the primarily oblique mus- 

 cular articulation on the distal face of Z 2 is, on the interpolated repeti- 

 tions, transformed into a straight muscular articulation. This articulation 

 would normally be oblique muscular when considered as the distal articula- 

 tion of a Zj ; but, considered as the proximal articulation of the following 

 Zi, it is, of course, straight muscular; whenever an articulation is morpho- 

 logically both straight and oblique muscular, the former, being dominant over 

 the latter, is always found. 



