30 UINTACKINUS: ITS STEUCTUEE AND EELATIONS. 



Pelmatozoa, on phylogeiietic principles, in which he subdivides the Class 

 Crinoidea into two sub-classes : Monocyclica and Dicyclica. 



The validity of such a division of the Inadunata was combated and 

 denied in the Monograph of the Crinoidea Camerata, upon grounds which 

 it is not necessary to restate here. There was undoubtedly much plausi- 

 bility in the suggestion of these two divisions, more as to the Inadunata 

 than to the Camerata. What made it especially attractive was the fact 

 that it was based upon differences in the primitive elements of the 

 Crinoid organization, representing phylogenetically different early stages 

 of the only Crinoid whose embryology we know. And the argument 

 which was considered by its author to be conclusive, was the assumed fact 

 that there was no such thing as a transition from one form of base to the 

 other. 



What, then, is the significance of the present discovery in relation to 

 this question ? It presents a difficulty far more formidable than the case 

 of the Reteocrinidas. 



For those are Lower Silurian types, — among the earliest known Crinoids; 

 and it is quite possible to suppose, if the Crinoids diverged into two lines of 

 development on this character, that they represent stages somewhere near 

 the point of such divergence. If the two forms of base represented by text 

 figures 1 and 2 had been found in specimens otherwise separable, they 

 would, under Mr. Bather's arrangement, have been unquestionably referred 

 to different genera, families, orders, and sub-classes. Considering the appar- 

 ent identity of these forms in every other point of structure, coupled with 

 their mode of occurrence and association, I do not see how any such sep- 

 aration can possibly be made in this case. We therefore have apparently 

 to deal with a case of individual variation, as to this supposed primitive 

 character, within the limits of a species. That is to say, in this species, 

 living in the same locality, having the same environment, floating in the 

 same mass, certain individuals matured to represent one stage of larval 

 development, i. e. with infrabasals, and others in another stage, i e. with 

 basals only. 



In short, they are the two supposed distinct types, Monocyclica and 

 Dicyclica, occurring in both young and adult of one and the same species. 

 It will not do to say that the species is dicyclic, but in certain individuals 

 the infrabasals are not developed, or are hidden by the centrale, or have 

 disappeared by atrophy. If this were so, the centrale oui^ht to be inter- 



