124 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



greater degree than do the pinnules on the inner arms, which are of a more 

 generalized type. 



Since the specialized specific characters are most pronounced on the outer arms 

 on each IIBr series, which thereby depart most widely from the primitive type, 

 it is a logical assumption that these outer arms represent a phylogenetical advance 

 over the inner arms. It has already been mentioned that they commonly bear an 

 additional axillary. 



Further evidence of the advance in development of the outer arms over 

 the inner is found in the curious Comatula etheridgei (part 1, fig. 78, p. 131), 

 which may possibly be the young of C. rotalaria. In this species the outer arms 

 arising from each IIBr axillary are always much smaller than the inner and always 



show evidence of undergoing rapid growth. Thus 

 in this form the outer arms are ontogenetically 

 of later development than the inner arms; and it 

 may be assumed that, maturing at a much later 

 period, they will exhibit any features character- 

 istic of the species in a more perfected form than 

 the inner arms. 



Therefore, in 20-armed species with four arms 

 on each postradial series the evidence seems to 

 show that the two outer arms are more advanced 

 than the two inner (figs. 176, 177, p. 89). 



In Lamprometra protectus and in Pontio- 

 metra andersoni, when 40 arms are present, eight 

 on each postradial series, it is noticeable that, 

 whereas the proximal pinnules on the outer side 

 of the outermost arms are the most developed, 

 those on the outer side of the innermost derivative 

 from each IIBr series are much more developed 

 than the arms between these and the outermost 



arms (the intermediate 30-armed stage is shown 

 FIG. 196. LATERAL VIEW OF TYPE SPECI- f -^g p gg\ 



MEN OF MARIAMETBA TUEEBCULATA. . . . . . 



Tracing the relationship still further, it is 



easily ascertained that while the phylogenetic status of the arms on postradial 

 series with four ultimate branches (the number 1 representing the most perfected 



type) is 



1 : 2 :: 2 : 1 



and on postradial series with eight ultimate branches 



1 :3 ::4 :2 :::2 :4 :: 3 : 1, 

 on postradial series with 16 arms it is 



1 : 5 ::7 : 3 :::4 : 8 ::6 :2 ::::2 :6 :: 8 :4 ::: 3 :7 :: 5 : 1, 

 each group of four arms tending to act as an independent unit. 



