Camerate Monocyclic Crinoids. 209 



left posterior basals (Nbs. 5 and 1 in the figure) and the shifting of 

 the right posterior suture as under 7. If this be Mr. Wilson's 

 meaning, lie is mistaken in thinking that I do not agree with it ; but 

 to his criticism of my supposed views I shall return later. 



" 10. The hexagonal, tripartite base of the Hexacrinidae resulted 

 from the interpolation of an anal plate by portional migration, 

 through shifting of the right-posterior basal suture to a posterior 

 position, and closure of the anterior and left-posterior basal sutures, 

 in a simple platycrinid with a pentapartite base." 



"11. The bipartite, hexagonal base in the Hexacrinidae resulted 

 from interpolation of the anal plate by portional migration, shifting 

 of the right-posterior basal suture to a posterior position, and 

 closure of the right-anterior basal suture, in a platycrinid with 

 a pentagonal, unequally tripartite base." 



["right-anterior" seems to be a misprint for " left-anterior".] 

 These two conclusions, apart and in conjunction, form the most 

 novel part of Mr. Wilson's paper, and he has made out a fair if not 

 absolutely convincing case. Wachsmuth & Springer thought that the 

 tripartite Hexacrinus base (Fig. 6) was derived from a tripartite base 

 of Platycrinus type (Fig. 3) by intercalation of the anal, swinging 

 round of the right posterior suture to the sagittal axis, as in 

 Batocrinidae, and widening of the left anterior basal (numbered 2). 

 Mr. Wilson calls this a " torsion theory", and objects to an enlarge- 

 ment elsewhere than adjoining the right posterior suture ; or, 

 discussing an alternative hypothesis [where his meaning is perhaps 

 obscured by another misprint] he says the theory "demands the 

 reappearance " in Hexacrinidae " of the right-anterior basal suture " 

 lost in Platycrinidae. Perhaps Mr. Wilson, here as elsewhere, does 

 not attach enough importance to the general symmetry imposed by 

 mechanical stresses, or to the selection-value of a stronger disposition 

 of sutures. If, as he concedes, it was possible for the right posterior 

 suture to shift 72° under certain stimuli, why should it be so 

 impossible for the anterior suture to shift an equal amount under 

 other stimuli ? Mr. Wilson, however, prefers to derive his 

 Hexacrinid base from the quinquepartite base of a primitive 

 Platycrinid (Fig. 1), and to imagine it as composed of posterior (5) 

 and left posterior (1) basals fused, left (2) and right anterior (3) 

 fused, and right posterior (4) enlarged. He is entitled to his 

 hypothesis, but it is still open to the objection which I raised in 

 1900, that no Silurian form is known which could link the imaginary 

 5-basalled Platycrinid with the Devonian Hexacrinid. 



The bipartite base of Dichocrinus and other Hexacrinidae (Fig. 9) 

 was regarded by Wachsmuth & Springer as derived from the 

 tripartite Hexacrinid base (Fig. 6) by the fusion of the compound 

 plate on the left (5 + ] ) with the small (left anterior) basal (2), and 

 by the shifting of the anterior suture back again into the sagittal 

 plane. This seemed plain sailing, but Mr. Wilson again objects that 

 "the pressure from the growing hind-gut . . . cannot affect the 

 anterior basal sutures". Granted ; but, as before, why overlook the 

 mechanical stimuli? "Furthermore, the examples of [abnormal] 



DECADE VI. — VOL. IV. — NO. V. 14 



