NOTES ON ECHINODERM MORPHOLOGY. 317 



of the upper arm-plates. Fewkes says on p. 145 : "lam led 

 to suppose that the dorsals^have been inadvertently omitted in 

 certain of the figures of a young Amphiura by Ludwig (pi. 

 xi, figs. 21 , 25), for he has not represented these plates in a 

 young specimen in which three pairs of side arm-plates are 

 represented (pi. xi, fig. 21, ad 3 ., ad*., ad' 3 .). 1 In a young 

 Amphiura of about the same age (pi. iii, fig. 19) 2 at least one 

 dorsal plate is formed, and in another as old as that repre- 

 sented in his fig. 25 (same plate) the dorsals have increased in 

 number. 3 In none of Ludwig's figures are dorsals represented, 

 though in figs. 21, 25, they must have been already formed." 

 Fewkes makes substantially the same statement in his con- 

 cluding summary on p. 147, " Dorsals are omitted in all 

 Ludwig's figures of the arm from the abactinal side. My 

 figure is younger than his (pi. xi, fig. 21), in which a dorsal 

 ought to be represented/' 



It was, however, expressly noted by Ludwig 4 that "In 

 Stadien welche nicht alter sind als das in fig. 21 gezeichnete, 

 sind noch gar keine Dorsalplatten vorhanden, obgleich schon 

 drei freie Armglieder angelegt sind." 



This passage must have altogether escaped Fewkes's notice, 

 or he would otherwise have scarcely have hinted at an inad- 

 vertent omission on the part of Ludwig, or have written so 

 positively as to what plates ought or ought not to be repre- 

 sented in a particular developmental stage ; while he makes no 

 reference on his own part to the differences in the time of 

 appearance of the radial shields, which are revealed by a com- 

 parison of his own observations with those of Ludwig, a fact 

 which may eventually turn out to be of very considerable 

 interest. 



1 This is copied as fig. in of the present communication (p. 312). 



2 Fig. iv, on p. 314. 



s There is something wrong about this comparison. For Ludwig's fig. 25 

 represents a younger and not (as Fewkes implies) a later stage than fig. 21. 

 There are not likely to be more dorsal plates developed in a form with two 

 adambulacrals (fig. 25) than in one with three (fig. 21). 



1 Loc. cit, p. 190. 



VOL. XXVIII, PART 2. NEW SER. V 



