UINTACRINUS: ITS STRUCTURE AND RELATIONS. 31 



radial in both cases ; whereas, as already shown, its orientation is reversed 

 from one to the other, precisely as in typical monocyclic and dicjclic forms. 



Such a condition 1 believe to be unique among the Crinoids. P. H. Car- 

 penter * identified the infrabasals in the adult of four species of Ophiura, 

 belonging to as many different genera, each of which embraces other species 

 not possessing infrabasals. But never before, so far as I know, have the 

 two forms of base been found in the same species. If Form D were an 

 isolated occurrence, it might be treated as a case of sport, indicating a 

 tendency to reversion to a dicyclic ancestor; but here the variation ex- 

 tends to nearly fifty per cent of the specimens, and we cannot say positively 

 which is the normal form. At Locnlity No. 1, Form M predominates; but at 

 Locality No. 2, Form D is largely in excess. 



If it were a Pre-Silurian fossil, we might believe that we had found the 

 point of incipient divergence of the two types; but here it occurs after the 

 two have long passed their culmination. Nowhere before have we found 

 any positive evidence of a transition, or direct evolution, of Dicyclica from 

 Monocyclica, or vice versa ; Bather and Wachsmuth and Springer are agreed 

 on that point. But here we have a case of direct and immediate descent 

 of one from the other, — though which way it was, we do not know. The 

 direct progenitor of the monocyclic and dicyclic individuals of these colonies 

 must have belonged to one type or the other. Hence it is that while the 

 Reteocrinidte might perhaps be disposed of by ignoring the family, and 

 assigning their genera to other families of Monocyclica and Dicyclica 

 respectively, such a course cannot be adopted here. Some other explana- 

 tion must be sought of the fact that characters which seem to distinguish 

 some of the largest divisions of the Crinoids are here found united within 

 the limits of a single species. 



It is difficult to frame an explanation of the co-existence of the two forms 

 of base in these Crinoids. The plates composing them are extremely thin. 

 There is no evidence of any kind of secondary growth or fusion m or about 

 the centrale in the monocyclic form. There is no reason, based upon any 

 characters exhibited by these specimens, for any inference as to one being 

 prior in development rather than the other, unless it is the greater preva- 

 lence of Form D among the young individuals. If one was derived from 

 the other during the life of the individual, it could only have been by one 

 of three methods, viz.: (1) the resorption of the infrabasals in the dicyclic 



* Quarterly Jouru. Microscop. Sci., N. S. XXIV., Jan., 1S84, pp. 1-23. 



