76 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



species is sufficient to result in the reduction of the number of underbasals from 

 4 or 5 to 3, we may readily infer that the much greater degree of speciahzation of 

 ^1. bifida over A. mediterranea would result in the elimination of underbasals entirely 

 from the ontogeny of the former. I can see no reason whatever for doubting the 

 accuracy of the work of Wy\'ille Thomson, Perrier, and the two Carpenters, who, 

 none of them, found underbasals in Antedon bifida, and I should be greatly sur- 

 prised if anyone in the future should find them in that species or in ^4. petasus, 

 except perhaps in sporadic instances." 



The same view is pronounced a little more cautiously in A. H. Clark's 

 monograph (page 316). 



This whole reasoning looks very interesting and ingenious, but it is, 

 nevertheless, quite untenable. Antedon bifida has indeed very well developed 

 infrabasalia, in spite of the fact that Allman, Wyville Thomson, Carpenter, 

 and A. H. Clark himself have failed to see them. On dissolving the young 

 Pentacrinoids under the microscope by means of dilute hypochlorite of 

 sodium, it is very easy to see the infrabasals, generally two larger and one 

 smaller plate, lying in the usual place, around the upper stalk-joint. Even 

 in Pentacrinoids so far developed that the arms have begun to branch, they 

 are still distinct. And it is decidedly not in sporadic instances that they 

 occur; I have found them quite constantly developed. 



In the light of this fact it is easily seen that the plate which Allman " 

 regards as the centrodorsal, in reality represents the infrabasaha; in the stage 

 figiired by Allman (the radials have just appeared) the upper stalk-joints 

 are still in new formation, so there is no centrodrosal as yet. Also in Wyville 

 Thomson's figure 1 of plate xxvii, the plate termed a represents the infra- 

 basalia, not the centrodorsal, as stated by WyviUe Thomson.^^ This has 

 already been correctly interpreted by MacBride, who, in the reproduction 

 given of the said figure in his "Text-book of Embryology," i, page 555, 

 designates it as under basal plates (SB), without otherwise saying anything 

 about it.«* 



After this I have no doubt that also Antedon petasus will prove to have 

 infrabasalia. The fact that there was no trace of infrabasalia in the Penta- 

 crinoid of this species figured in my "Report on the Echinoderms of North- 

 east Greenland (page 251, plate x, figure 3), does not prove anything about 

 it, as it would hardly be possible to see these plates at so late a stage without 

 dissolving the calyx, which was, of course, not done, this being the only 

 specimen known till then." 



" Allman. On a pre-brachial stage in the development of Comatula. Trans. R. See. Edinburgh, xxiii, 

 p. 244, 1863. 



" This figure, as also the other figiu-es in Wyville Thomson's memoir, gives upon the whole a very rough 

 and inaccurate representation of the calcareous plates. 



s« Evidently by a lapsus calami, MacBride states this figure (as also figure 408) to be "after Carpenter" 

 instead of Wyville Thomson. 



f^ Recent researches have given the result that Antedon petasus has really infrabasaha (comp. the 

 author's paper, "Notes on the development and the larval forms of some Scandinavian Echinoderms." 

 Vid. Medd., vol. 71, 1920, p. 153). 



