Mr. F. A. Bather on British Fossil Crinoids. 319 



B. The Anal Plates. 



We have now to consider the character that forms the 

 basis of Wachsmuth and Springer's classification, viz., the 

 number and relations of the plates in the posterior inter- 

 radius. And first it may be pointed out that, as interradials 

 do not enter into the composition of the dorsal cap in any 

 Fistulate, none of these plates can well be the homolognes of 

 interradials : in many of the Camerata actual interradials 

 are present in the anal area, but in the Fistulata at least we 

 must look elsewhere for the origin of the so-called " anal " 

 plates. 



Physiologically these plates have a two-fold importance. 

 Their chief function is to actually support the ventral sac : 

 in Gatillocrinus (PI. XIV. fig. 29), for example, the "anal 

 plate " is on a level with the arm-joints and supports a single 

 row of large and heavy plates ; in loerinus (PI. XIV. fig. 5) 

 the " anal plate " merely serves to support a line of stout 

 plates which form a median ridge to the ventral sac. Their 

 second function is to increase the space between the right and 

 left posterior radials, so as to allow room for the free deve- 

 lopment of the sac : of this Euspirocrinus (PI. XIV. fig. 17) 

 and Poteriocrinus (PI. XIV. fig. 26) are good examples. This 

 physiological aspect of the question is noteworthy, because it 

 assists our comprehension of the morphological relations of 

 the plates. 



The main types of structure are familiar to all students of 

 Palaeozoic Crinoids ; but for convenience of reference I have 

 given diagrams of the composition of the dorsal cup in every 

 genus of the Fistulata (PI. XIV.). These diagrams, which 

 are compiled from the best authorities and verified when 

 possible by reference to actual specimens, will also serve to 

 draw attention from the confused masses of verbiage that 

 have somewhat obscured the subject, and to concentrate it on 

 the facts themselves. To further dispel prejudice, the plates 

 as to which any doubt exists are, in the main diagrams, left 

 unlettered ; but they are repeated alongside with the various 

 interpretations that have been or that may be placed upon 

 them. The more important variations are those presented by 

 Hoplocrinus (1 & 2), Ifybocrinus (3), Baerocrinus (1), 

 loerinus (5), Heterocrinus (6), Dendroerinus (15), hlomo- 

 crinus (16), Cyathocrinus (20), Poteriocrinus (26), Botryo* 

 crinus (30), and Cerioerinus (38) ; and I would request any 

 not already well acquainted with the structure of those genera 

 to study the diagrams of them before proceeding. 



Fluctuations of opinion have rendered this subject so per- 



