no. 1636. ARM HOMOLOGIES l\ RECENT CRINOIDS— CLARK. 129 



erinus, in its young stage corresponding to the ten-armed condition 

 of Isocrinus, has but five (inns; consequently we await with more 

 than usual interest the discovery of the very young of Metacrinus. 



This result of the analysis of the arms in Isocrinus and Metacrinus 

 raises the question, are they really so different as is commonly sup 

 posed? Is the separation of Isocrinus and Metacrinus as at present 

 understood natural? Both these questions must be answered in the 

 negative. Metacrinus was separated from Isocrinus because of its 

 more numerous " radials," the homologies of the joints not being con- 

 sidered. Thus it appears that Isocrinus blakei, I. decorus, and /. 

 asteria (to which must be added /. riaresianus) are intermediate in 

 structure between Isocrinus wyville-thomsoni, I. parrce, 7. alterni- 

 cirrus, and /. sibogce, and the numerous species of the genus M < fa- 

 n-in us. The /. asteria group has r L x and Z L . united by synarthry, and 

 separated from the radials by a single interpolated series; extrane- 

 ous division occurs distal to Vj, ; the /. parrce group always have the 

 synarthries replaced by syzygies, and Z 3 and Z., always in the free 

 undivided arm, separated from the radials by a series of interpolated 

 divisions/ extraneous divisions never occur. Metacrinus has Z x the 

 first post-radial joint, no interpolated series, but all the arm divisions 

 extraneous/ the single possible synarthry is replaced by a s} r zygy. 

 The interrelations of Metacrinus, the Isocrinus asteria, and the /. 

 parrce groups, may be summarized as follows: 



Metacrinus (fig. 28). /. asteria (fig. 27). /. parrm (fig. 26). 



No synarthries. Synarthries present. No synarthries. 



Z, first post-radial joint. Z t third post-radial joint. Z, separated from the ra- 



dials by numerous inter- 

 polated series. 

 Extraneous division only. One interpolated series only ; All interpolated divisions. 



distal divisions extrane- 

 ous. 



It is plain that the Isocrinus parrce and /. asteria groups are as dif- 

 ferent from each other as Metacrinus is from the latter: and if Metct 

 crinus is to be recognized as a valid genus, the Isocrinus parrce and 

 /. asteria groups should also be kept separate. Treating these three 

 divisions as of equal value generically, it is interesting to find that 

 they fall into definite faunal areas, and occupy characteristic bathy- 

 metric altitudes. The three divisions, with the species in each as 

 now 7 understood, are as follows: 



a\ Zi and Z. the first lw<> post-radial joints, not repeated; all arm division 

 extraneous; second post-radial joint not an axillary, but bearing a pinnule; 

 basals very broad, forming, when viewed dorsally, a rounded pentagonal 



figure; infrabasals large and prominent Metacrinus (fig. 28) 



a s . Zi and Z 2 repeated at least once; the second post-radial joint an axillary. 

 b\ r Lx and Z,. the third and fourth post-radial joints: infrabasals present? 

 c x . One interpolated series only; basals broad, forming, when viewed dor- 

 sally, a rounded pentagonal figure; infrabasals? 



IIVI'AI.OC KIMS (<•;'. figs. II and 17 > 



"Infrabasals have been found in M. serratus by Doderlein, and in .1/. superbux, 

 and in several specimens (all dissected) of .1/. rotundus by Clarli, 

 Proc. N. M. vol. xxxv— OS 9 



