316 Mr. F. A. Bather on British Fossil Crinoids. 



A. The Base : whether Dicyclic or Monocyclic ; if the former, 



whether the Infrabasals are 5 or 3 in number. 



B. The Anal Area of the Dorsal Cup : the number of plates, 



from to 3 or possibly more, that are included ; the 

 relations of those plates to one another and to the adjoin- 

 ing plates. 



C. The Arms : whether simple, dichotomously brandling, or 



with lateral armlets ; whether pinnules are present or 

 absent. 



D. The Mode of Union between Plates and Ossicles : whether 



Syzygy, Close Suture, Loose Suture, or Muscular Arti- 

 culation prevails. 



In attempting to construct a natural classification care must 

 be taken not to fix on any one character to the exclusion of 

 others ; since, however, it would be impracticable to discuss 

 all the above points at once I shall proceed to deal with them 

 in the order just indicated. 



A. The Base. 



Our views as to the difference between Monocyclic and 

 Dicyclic forms have of late undergone considerable change, 

 chiefly owing to the discovery of minute infrabasals in either 

 the embryonic stages or the adult of many forms previously 

 supposed to be without them *. At the same time these dis- 

 coveries have confirmed the rule laid down by Wachsmuth 

 and Springer : — that where infrabasals are present the angles 

 of the column are interradial and the sides of the column, 

 the lobes of the axial canal, and the cirri are radial; but 

 that where infrabasals are absent these conditions are re- 

 versed j\ While therefore many forms turn out to be Pseudo- 

 Monocyclica, the distinction between true Monocyclica and 

 Dicyclica is if anything strengthened. 



To turn to the Fistulata. In the Hybocrinidas no infra- 

 basals have been observed ; but Wachsmuth and Springer 

 suggest (Rev. I. (74) Proc. 1879, p. 297) that those plates 

 are " probably rudimentary," and, as the simplicity of the 

 column precludes definite proof, we may have to consider the 



* See especially H. Bury, " The Early Stages in the Development of 

 Antedon rosacea,' 11 Phil Trans, clxxix. B (1888), pp. 257-301, London, 

 1889. Infrabasals on pp. 270-271, 288-289, pi. xlvi. fig. 46, pi. xlvii. 

 figs. 48, 52,53. 



t See this rule more fully stated in their paper " Discovery of the 

 Ventral Structure of Ta.rocrinus &c," Proc. Ac. Nat. Sci. Philadelphia, 

 1888, p. 351. 



