from the Devonian of CormcaU, 165 



Mr. J. W. Fewkes ^ on AmpJiiura squamata have shown that the 

 two lateral halves of each vertebra unite first at the proximal and 

 distal ends, leaving a space between. This is suggestive of an 

 origin from proximal and distal elements. On the other hand, each 

 lateral half springs from a single centre of calcification, from which 

 it stretches out in a distal direction, so that the incipient ambulacra! 

 element is thicker at its proximal end. This last-mentioned feature 

 is paralleled by the boot-shaped proximal portions of the ambulacral 

 ossicles in the Protasteridae. At a slightly later stage the 

 ambulacral of Amphiura broadens at the distal end, which thus 

 <5omes to resemble the distal portion of the Protasterid ambulacral. 

 If this distal portion in AmpMiira were, as indicated by Professor 

 Ludwig's observations, merely an extension of the proximal portion, 

 then it might be inferred that the whole structure, both here and 

 in the various Palaaozoic forms alluded to, represented a single 

 ambulacral pair and not two successive pairs. In such case one 

 would accept Dr. Gregory's suggested explanation (op. cit., p. 1032) 

 "that the smaller pieces are only triangular, distal portions of the 

 ambulacral ossicles, apparently separated from the proximal portion [sj 

 by a groove." Dr. Gregory further suggests that these grooves 

 or depressions " were for the lodgment of the ventral muscles which 

 ■moved the arms." A far more probable explanation surely is that 

 they were for the reception of the branches from the radial water- 

 vessel to the podia. This explanation, however, consonant as it 

 is with the facts of development in Amphiura, reminds one of 

 another difficulty. Professor Ludwig (op. cit., p. 185) has pointed 

 ■out that, whereas the so-called ambulacrals of Asterids lie at right 

 angles to the perradius and between the successive branches and 

 podia, those of Ophiurids lie parallel to the perradius and across 

 the branches to the podia. Now if, as we have every reason to 

 believe, the Ophiurids were derived from Asterids, it is hard to see 

 why this change should have taken place. That is the difficulty, 

 but it is a difficulty that disappears if we suppose that the vertebra 

 is really compounded of two successive ambulacral pairs in the 

 manner outlined above. The remaining objection to this latter 

 hypothesis is that, as Dr. Gregory has pointed out, it postulates the 

 suppression of alternate podia. Nevertheless, similar fusion and 

 concomitant suppression are not so unknown among Echinoderms 

 as to render this objection a fatal one. 



The suggestion that the vertebrae of Ophiurids are 

 •composed of two successive ambulacral pairs certainly 

 cannot be proved with the evidence at our disposal. But it is 

 doubtful whether it can be disproved, and I therefore make it to 

 promote a search for further evidence pro or con. 



Two other features in the present specimen need discussion, and 

 may perhaps throw light on the problem just posed. The first of 

 these is the fact that the broadest and stoutest portion of each 

 arm-segment appears to be the distal half (marked a in Fig. 1), 

 whereas in the developing Amphiura and in the adult Protasteridse 



1 Bull. Mas. Comp. Zool. Harvard, vol, xiii, No. 4 ; 1887. 



