URASTERELLIDtE. 129 



assertions that all the Asterozoa can be derived from Hudsonaster, makes the same 

 point, for he states (85, p. 49) with respect to accessory disc-pieces : " In mature 

 Hudsonaster they form a single ring, but in the young of the cryptozonian Uras- 

 terella (U. ulrichi) and in mature Gallia sterella there are none of these accessory 

 pieces present. For these reasons it is thought that in the Ordovicic there will be 

 found a small asterid, even more primitive than Hudsonaster, that will be devoid of 

 accessory disc pieces." 



Some interesting correspondence with Dr. Gremmill enables me to cite the 

 presence of a single primary interradial as evidence of the exceedingly primitive 

 nature of the arrangement of the disc-ossicles. Two years ago I wrote to him and 

 pointed ont the seeming importance of the five large primary interradialia in the 

 Hudsonasteridae as being paralleled in the young stages of the Recent Asterias. In 

 his reply he remarked : " The large regular interradials in the young Asterias are 



Text-fio. 81. —Plan of ossicles on the apical surface of a young individual of Ura.slerella thraivemis (D. 46). 

 C, centrale ; I.M., infero-marginalia ; li., radialia ; X., first adradial. x 10. 



no doubt of much importance for purposes of comparison, but only, I should say, 

 within forms of definite Asteroid or Ophiuroid type. These ossicles are, of course, 

 as a series, results and not precursors of perfected quinqueradiate symmetry, and 

 there is reason, I think, for considering that one of them (that in the madreporic 

 interradius) is not strictly homologous with the rest" (see 94, p. 266). At that time 

 I was not acquainted with the structure of Urasterella, and it is pleasant to observe 

 that the structure of the genus substantiates his view, that originally the only 

 interraidial present was that which throughout the whole of the Asteroidea is 

 closely associated with the madreporite. 



The remaining plates of the disc are figured as radialia, adradialia and infero- 

 marginalia. This is not quite in accordance with the views of Schuchert, who 

 regarded the Urasterellidae as belonging to the " Cryptozonia," that is, to the 

 Asteroidea in which, in the adult, the original marginal series can no longer be 

 clearly distinguished from the remaining plates. He was only prepared to admit 

 that he had been able to recognise infero-marginalia in the axillary regions of a 



