CRINOIDEA. 



423 



general structure of the " tegmen calycis " in the great Palaeozoic 

 family of the Poteriocrinidce is the same as in the Cyathocrinidce, but 

 both the interradials and the " oral plates " are often inconspicuous, 

 or imperfectly developed. In these forms, also, the excentric anus 

 is prolonged into a proboscidiform tube, which is often of great 

 size (fig. 298). 



As regards the classification of the Crinoids, the first well-ground- 

 ed systematic arrangement was that proposed by Johannes Miiller, 

 who divided the order into the three 

 sections of the Tessei/ata, the Articu/ata, 

 and the Costata. The first of these 

 sections included the Palaeozoic Crin- 

 oids, and its name was based on the 

 fact that in these the plates of the 

 calyx are united by sutures which do 

 not admit of movement. The section 

 of the Articulata, again, included the 

 living, Tertiary, and Secondary Crin- 

 oids (Marsupites alone excepted), in 

 which the calycine plates are mov- 

 ably articulated ; while the section Cos- 

 tata comprised only the aberrant Ju- 

 rassic genus Saccocoma. Zittel, in his 

 admirable ' Handbuch der Palaeon- 

 tologie,' has followed Miiller's classi- 

 fication, with various emendations and 

 modifications. Miiller's classification, 

 though a great advance upon that pro- 

 posed by his predecessors, cannot be 

 considered, even as modified by Zittel, 

 to be a strictly natural one, since forms 

 which agree in all the other main points 

 of their organisation may differ as re- 

 gards the mode in which the calycine 

 plates are joined together. Thus the 

 family of the Ichthyocrinidce, though 

 closely resembling the other Palaeozoic Crinoids in its essential or- 

 ganisation, is characterised by the possession of movably articulated 

 radial plates, and thus should properly fall under the section of the 

 Articulata. The classification which will be here followed is that 

 adopted by Dr P. Herbert Carpenter and by Mr Charles Wachs- 

 muth, in which the Crinoidea are divided into the two primary 

 sections of the Palceocrinoidea and Neocrinoidea. In the division 

 of the Palcsocrinoidea are comprised all the known Palaeozoic 

 Crinoids, and the distinguishing characters of the division are, 



Fig. 298. — Calyx and part of the 

 arms of Poteriocrinics radiatus, show- 

 ing- the proboscidiform anal tube. 

 Carboniferous. (After De Koninck 

 and Le Hon.) 



