Knr.u in 



< 'K I NOIDEA FISTULATA 151 



by for. <>nik. Arms free from tlu radial* >/////v//v/ .- 

 ,.tii>imllii /'/'s/r/W. /'i /i miles present or absent. Ordovician to Trias. 



The /Y.-7//A//" an- characterised by an enormous development of the posterior 

 interradius, which is extended upward s< as to form a large ventral sac or tube: 

 The plates of this sac are in some cases perforated by small, round, or slit-like 

 p<ivs (I'l'sjiiriifiti'ii jutffa) while in others the pores are replaced by superficial 

 pits. In the latter forms, a large, profusely perforated plate (madreporiti) is 

 interposed <>n the disk proper between the sac and the month. 



The position of the anus, as observed by Wachsmuth and Springer, is at 

 the side of the sac nearest the mouth ; or it 

 pierces the disk proper between the sac and 

 the mouth. 



The radials in some families of the Fistulata 

 are transversely bisected in one, two, or in three 

 rays. \Vhen three of the compound radials are Fi( . 205 



present, they are generally distributed in the Diagram shown.- arrangement of plates 

 right posterior, the anterior, and the left antero- ^^^^^^^^\ 

 lateral rays; but when only one radial is , ^ a", Anal ami lower tnbe plates (after 

 bisected, it is constantly that to the right of 



the anal area. The phases exhibited by the last-named plate in its palaeonto- 

 logical development furnish excellent differential characters. The superradial, 

 or arm-bearing portion of the plate, is situated in the earlier forms directly 

 in line with the inferradial or lower part of the plate ; but in later forms 

 it is pushed to the right by the gradual increase in width of the ventral sac. 

 The inferradial remains constant in position, but when supporting the sac, as 

 is usually the case among the later forms, it receives the name of radiancd. 

 Primitively, however, as was shown first by Wachsmuth and Springer, and 

 subsequently by Carpenter and Bather, the radianal represents the lower 

 portion of the right posterior radial ; and it has, therefore, nothing in common 

 with the anal plate, which is a specialised interradial. 



Under the Fistulata are included the following families : Hybocrinidae, 

 A nomalocrinidae, Heterocrinidae, Belemnocrinidae, Catillocrinidae, and Calceocrinidae, 

 comprising the monocyclic forms ; and Gasterocomidae, Cyathocrinidae, Poterio- 

 cr in ii fiic [Encrinidae], and Agassizocrinidae among the dicyclic. 



Family 1. Hybocrinidae. Zittel. 



Base mononjclic. Bsals foe, high. T1ie % right posterior radial compound ; the 

 inferradial supporting the ventral sac ; the superradial extremely small or undeveloped. 

 I 'en fnil ,sv//; in its most primitive foi'm, extending but little above the rest of the legmen. 

 ( )rdo\ ician. 



ffybocrinus, Billings. Calyx cup -shaped or obconical. Inferradial large, 

 angular above ; supporting to the right a very small superradial, and to the 

 left the first tube-plate, which generally resembles the superradial in form and 

 size. It with a crescent-shaped facet. Arms simple, devoid of pinnules, and 

 composed of quadrangular joints. Ordovician ; Canada, Kentucky, and 

 Tennessee. 



II'ijilncAnm, Grewingk (Fig. 256). Like the preceding, but with the 

 inferradial sloping only to the right, and supporting a small, trigonal super-. 



