258 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



The observations of Prof. Ludwig 1 and Mr. J. W. Fewkes 2 on 

 Amphiura 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 center of calcification, from which it stretches out in a distal 

 direction, so that the incipient ambulacral 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 Prot- 

 asteridas. At a slightly later stage the ambulacral of Amphiura 

 broadens at the distal end, which thus comes to resemble the distal 

 portion of the Protasterid ambulacral. If this distal portion in 

 Amphiura were, as indicated by Prof. 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 Paleozoic 

 forms alluded to, represented a single ambulacral pair and not two 

 successive pairs. In such case one would accept Dr. Gregory's 

 suggested explanation 3 'that the smaller pieces are only triangular, 

 distal portions of the ambulacral ossicles, apparently separated from 

 the proximal portion[s] by a groove.' Dr. Gregory further sug- 

 gests that these grooves or depressions 'were for the lodgment of 

 the ventral muscles which moved the arms. 7 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, how- 

 ever, consonant as it is with the facts of development of Amphiura, 

 reminds one of another difficulty. Prof. Ludwig 4 has pointed out 

 that, whereas the so-called ambulacrals of Asterids He 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 Ophi- 

 urids 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 can not be proved with 



1 Zeits. wiss. Zool., vol. 36, 1881, p. 181. 



2 Bull. Mus. Comp. ZooL, vol. 13, No. 4, 1887. 



Proc. Zool. Soc. London for 1896, 1897, p. 1032. 

 * Op. cit., p. 185. 



