150 Clarli — Tiro New Orinoid Genera. 



sibogse and J. wyville-ihomsoni arm reduplication is effected by 

 the interpolation of various division scries which in reality are 

 mere repetitions of the first two joints of the free undivided arm, 

 as in multibrachiate comatulids, in Metacrinvs the arm redu- 

 plication results from a splitting of the arm at more or less un- 

 certain intervals.' The former I call interpolated division, the 

 latter extraneous division. In Isocrinus asteria, I. blakei, and /. 

 decorus, we find a combination of these two modes of arm divi- 

 sion; the first arm division (and the first only) is an interpo- 

 lated division, those following are extraneous divisions. In Meta- 

 crinus and these two divisions of Isocrinus, the first post-radial 

 joints bearing pinnules are in all eases homologous, regardless of 

 tbe numberof joints intervening between them and the radials; 

 this is shown by the articulation between them and the succeed- 

 ing joint being a muscular articulation in which the transverse 

 ridge separating the dorsal ligament fossa from the interarticular 

 ligament fossa' is strongly oblique; that is, an oblique muscular 

 articulation, as opposed to a muscular articulation where the 

 transverse ridge is at right angles to the dorso-ventral axis 

 of the joint face, or a straight muscular articulation. The first 

 oblique muscular articulation occupies morphologically the same 

 position in the arms of all comatulids, and in all the recent Pen- 

 tacrinitidae, and is followed exclusively by articulations of the 

 same type, interspersed with occasonal sj'zygies; proximal to 

 tbe first oblique muscular articulation, only straight muscular articu- 

 lations and synarthries or bifascial articulations ( which may in any 

 or all cases be replaced by sipi/i/ics) occur. The first oblique mus- 

 cular articulation in Metacrinus and the two divisions of Isocrinus 

 is always found on the distal end of the joints bearing the first 

 pinnule. 



The Isocrinus asteria group is, in arm structure, intermediate 

 between the /. parrse group in which only interpolated division 

 is found, and Metacrinus, which has only extraneous division, as 

 it has one interpolated series, followed by one or more e.rtraneous 



* In both sections of Isocrinus, as in the multibrachiate comatulids, the young are 

 ten-armed, the multibrachiate condition arising, as explained by Minckert, through a 

 process ofautotomy by which ihe original arms break off at the articulation between 

 tin' first and si 'cone I brachials ni' i in 1 undivided arm, or the third ami fourth post-radial 

 ji lints. Now in M< tacrinus tin- li i si two post-radial joints correspond to these first two 

 post-costal joints in young Isocrinus; therefore, ii is reasonable to suppose that the very 

 young of Mi ' I tin- in a x. ha ving but five articulations when autotomy is possible instead of 

 ten as in TsocrtrwCs, will be found to possess but five arms. 



