166 



ECHINODERMATA PELMATOZOA 



SUB-KINGDOM III 



present in nepionic, frequently also in the adult stages. Plates of the dorsal cup 

 heavy ; basals and radials united by dose suture; radials and lower brachials by 

 muscular articulation or by syzygy. Radials laterally in contact, except in Thaumato- 

 crinus ; but small irregular plates are frequently interposed between the costals and 

 distichals. Anal plates represented only in the larval stages. Arms uniserial and 

 pinnulate, the ossicles pierced by a dorsal canal. Trias to Eecent. 



The Articulata include all Kecent, Tertiary, and Mesozoic Crinoids, with the 

 exception of Marsupites, Uintacrinus [and Encrinus, according to Wachsmuth 

 and Springer]. They are chiefly characterised by the exposed condition of 

 the mouth and food-grooves, as well as by a canal which perforates the arms. 



This is known as the dorsal or axial canal, and 

 contains a nerve-band and fibres of connective 

 tissue. Starting from the dorsally situated 

 chambered organ (Fig. 276), there are five canals 

 which traverse the basals as far as the centre, 

 where they divide into two branches which con- 

 tinue upward through the radials and brachials ; 

 and there is also in the radials a ring -canal 

 which serves to connect the longitudinal canals 

 with one another. In young individuals these 

 canals are in the form of open grooves on the 

 inner side of the plates, but they become per- 

 fectly closed in the adult condition. 



Diagram showing course of axial canals [ T tne 4rtwulata (Articulosa) Wachsmuth 



in the calyx .and arms of Encrinus. Canals an( j Springer refer only the Pseudomonocuclica ; 



represented by dotted lines when pene- . r & J 



trating the interior of the plates, and by that IS to Say, L/rmOlds which are Constructed 



heavy lines when exposed on the inner ,v j- i- i i , r r ,r r 



side of the calyx (after Beyrich). on the dicychc plan, but in which the mfra- 



basals are rudimentary, and are more or less 



completely fused with the top stem- joint, and in which the last-named joint 

 is not the youngest in the stem, as is the case in all other forms. The 

 families thus embraced are the Apiocrinidae, Bourgueticrinidae, Eugeniacrinidae, 

 and Comatulidae among Mesozoic and later Crinoids, and the Ichthyocrinidae 

 among the Palaeozoic. The Ichthyocrinidae, which are devoid of pinnules, 

 are placed in the sub-group Articulata Impinnata ; the others, which are all 

 pinnulate, constitute the sub-order Articulata Pinnata. The Pentacrinidae and 

 Encrinidae, both of which are in all probability derived from the Poteriocrinidae, 

 are assigned by these authors to the Fistulata, and the same is also true of 

 the Plicatocrinidae and Holopidae, in which the top stem-joint is the youngest 

 joint of the column wherever the latter is represented.] 



FIG. 276. 



Family 1. Encrinidae. Roemer. 



Dorsal cup low, saucer-shaped, with dicyclic base. Infrabasals five, very small, 

 and covered by the upper stem-joint. Basals five, large ; radials five, the articular 

 faces truncate, and provided with transverse ridges. Interbrachials absent, legmen in 

 the form of a vault. Arms 5 x 2 or 5 x 4 ; heavy and simple, closely abutting, and 

 either biserial or composed of alternately arranged cuneate joints. Column round, 

 rarely giving off cirri ; the terminal end thickened and laterally extended. Trias. 



Encrinus, Miiller (Figs. 276-278). R succeeded by two costals, of which 



