STRUCTURE OF THE CRINOIDE^. 13 



Dendrocrinus, Porocrinus, Carahocrimts, Rliodocrinus, and others. In 

 Ghjptocrimis, Helerocrimis, Thysanocrinus, Hyhocrinus, and Cleiocrinus, 

 there are no sub-radials, the rays springing immediately from the 

 basals. 



V. The Rays and Radial Plates. 



In all Crinoids there are five rays, the lower plates or extremities 

 of which are included in the structure of the cup and form part of 

 the shell, while the upper portions are prolonged above the body, 

 and constitute the arms, which are generally free and more or less 

 branched. In Rhodocrmus and Glyptocrijius each ray consists at 

 first of a single series of three plates, sometimes called the primary 

 radials or simply the radials; it then divides into two series, called 

 the secondary radials. In these two genera the primary and secon- 

 dary radials enter into the composition of the cup. In Glyptocrinus 

 the first or lowest primary radials rest upon the upper edges of the 

 basal plates, alternating so that each ray is supported by the conti- 

 guous sides of two of the basals. In Rhodocrinus there is a series 

 of sub-radials between the basals and primary radials. In such 

 genera as Palaocrimcs, Carahocrinus, Dendrocrinus, and Porocrinus, the 

 first primary radial only is included in the walls of the cup, but the 

 second plate and all above it are free. In the very remarkable 

 genus Cleiocrinus the primary, secondary, tertiary, quaternary and 

 quinary rays are all firmly connected together, the free arms com- 

 mencing with the sixth or seventh division. 



The student will find many other modifications of the radial 

 system of the Crinoideas by consulting the works of various palaeon- 

 tologists ; but the above are the more common ones, and those most 

 prevalent in the Lower Silurian of Canada. 



VI. The Inter-radials. 



The divisional space between two rays is called an inter-radius ; 

 and as there are five rays, there must be of course an equal number 

 of inter-radii. Four of these are always of equal size, and are called 

 the "regular inter-radii," and when they contain plates these are 

 designated the "regular inter-radials." The fifth is larger than 

 either of the other four, and is called the "azygos inter-radius," from 

 the Greek azngos, "unyoked," or "not paired." The plates in this 

 inter-radius are called the azygos inter-radials. In most v/orks the 

 "azygos inter-radials" are termed "anal plates," but as they are 



