ORDER IIT CRINOIDEA—FISTULATA Ti. 
usually surrounded by five orals. Arms free from the radials upward ; wniserial, or 
exceptionally biserial. Pinnules present or absent. Ordovician to Trias. 
The Fistulata are characterised by an enormous development of the posterior 
interradius, which is extended upward so 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 
pores (respiratory pores); while in others the pores are replaced by superficial 
pits. In the latter forms, a large, profusely perforated plate (madreporite) is 
interposed on the disk proper between the sac and the mouth. 
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 Fistulatu 
are transversely bisected in one, two, or in three 
rays. When three of the compound radials are 
present, they are generally distributed in-the _ piagram showing arrangement of plates 
right posterior, the anterior, and the left antero- aie a CU Na ase 
lateral rays; but when only one radial is a, a’, a”, Anal and lower tube plates (after 
bisected, it is constantly that to the right of eee 
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 radianal. 
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 :—Hybocrinidac, 
Anomalocrinidae, Heterocrinidae, Belemnocrinidae, Catillocrinidae, and Calceocrinidae, 
comprising the monocyclic forms; and Gasterocoindae, Cyathocrinidae, Poterio- 
crinidae | Encrinidae|, and Agassizocrinidae among the dicyclie. 

Family 1. Hybocrinidae. Zittel. 
Base monocyclic. Basals five, high. The right posterior radial compound ; the 
inferradial supporting the ventral sac ; the superradial eatremely small or undeveloped. 
Ventral sac in its most primitive form, extending but little above the rest of the tegmen. 
Ordovician. 
Hybocrinus, 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. J? with a crescent-shaped facet. Arms simple, devoid of pinnules, and 
composed of quadrangular joints. Ordovician; Canada, Kentucky, and 
Tennessee. 
Hoplocrinus, Grewingk (Fig. 256). Like the preceding, but with the 
inferradial sloping only to the right, and supporting a small, trigonal super- 
