Camerate Monocyclic Crinoids. 211 



"by interstitial growth" (p. 502, also p. 497). The phrase is taken 

 from Wyville Thomson's account of the development of Antedon 

 (1865). Whether Wyville Thomson used the phrase (which he 

 qualified by "apparently") in its customary meaning or no, there 

 is no support for the idea. II; might, if true, have been useful in 

 explaining the changes in the basal plates, but Mr. Wilson makes no 

 further use of it. 



Mr. Wilson is strongly disinclined to believe that division of plates 

 has ever occurred as a process of normal development in Echinoderms. 

 Indeed, he stoutly affirms " that there are no known instances of the 

 bisection of a growing plate in modern Echinodermata " (p. 674). 

 This is a question of deep morphological importance. Presumably 

 Mr. Wilson has not made his assertion without ransacking the 

 literature ; and yet one would like to hear what the embryologists 

 have to say. 



In discussing possible methods of plate migration, Mr. Wilson says 

 (p. 539, I quote from a copy corrected by him) : "in Antedon . . 

 Promac\y{]ocrinus and Hathrometra . . . the anal and radianal, being 

 more firmly attached to the viscera than to the adjacent plates, are 

 bodily lifted out of the cup into the tegmen by the accelerated growth 

 of the hind-gut. This process is the one which undoubtedly explains 

 the migration of the radianal in all fossil crinoids." This explanation 

 may perhaps be correct for the recent crinoids mentioned, and it may 

 perhaps apply to the later evolutionary stages in the Dendrocrinoidea 

 ( Poteriocrinidae s. lat.) ; but to say that it is " the one " explanation 

 for " all fossil crinoids" overshoots the mark. In the first place the 

 observed ontogeny of the comatulids cited is too rashly applied 

 to the supposed phylogeny of various extinct genera in which 

 there is no actual proof of migration during individual growth. 

 After all, the view has been held that the ontogenetic changes in the 

 comatulids are a recapitulation of previous phylogenesis (Bather, 1893, 

 llecapitulation Theory in Palaeontology. Nat. Science, II, p. 275, 

 also. Ill, p. 238). The plates of the cup are not really " attached to 

 the viscera ", but lie in the integument. It is true that in the larval 

 Antedon the anal is not closely apposed to the adjacent cup-plates ; 

 but in palaeozoic crinoids there was a well-formed sutural union. 

 The growth of the hind-gut was no doubt the stimulus, but its action 

 can hardly have been a direct pull. The anal area first widened to 

 give it room, and the space was filled partly by a doivnward growth 

 of some plate above, partly by widening of the adjacent radials 

 and the posterior basal, partly by portion al migration (to adopt 

 Mr. Wilson's useful phrase) of that plate below the right posterior 

 radial which we call the radianal, sometimes even by development of 

 a supplementary plate below the radianal (e.g. Caraoocrinus). In 

 these cases the expansion of the radianal was in an adanal direction, 

 but usually along a horizontal line, sometimes even downwards into 

 the basal circlet (e.g. Carabocrinus, Strophocrinus, and Thenar ocrinus, 

 all overlooked by Mr. Wilson) ; it was no more dragged up than was 

 the upper anal dragged down. Sometimes the widening of the cup was 

 effected in a different manner; in the Pisocrinidae and Catillocrinidae, 

 for instance, the radianal has enlarged on the right side of the right 



