82 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



the axillary ; while in species with two costals the first pinnule is given off 

 from the second distichal. In the Heterocrinidae, in which the syzjgies pre- 

 vail throughout the whole arm, they are formed in most of the species by 

 more than two plates. While Epactocrinus grandk has but one hypozygal, 

 Heterocrimm has two or more, followed by the epizygal bearing the pinnule, 

 which in this group might be properly called an armlet. A similar arm 

 structure is found among the later Calceocrinidae, and also in the Belemno- 

 crinidae. In Epadocrinus grandis the syzygies are especially well marked, the 

 epizygal joints being wedged-shaped, the hypozygal quadrangular, and 

 arranged parallel to the distal faces of the preceding epizygal. 



In some groups of the Fistulata it is exceedingly difficult to discriminate 

 between pinnules and armlets. As a rule, pinnules are more delicate, and 

 are given off, except in cases of syzygy, from successive joints ; while most 

 of the armlets are arranged at more or less irregular intervals along the sides 

 of the ordinary arms, and are usually branching. We find armlets among 

 the HeterocrinidaB, Belemnocrinidae, in Bargcrinus, Botrgocrinus, and the later 

 Calceocrinidae. The Cyathocrinidae have long filiform arms, which frequently 

 dichotomize, but have neither armlets nor pinnules. The Anomalocrinidae 

 have branching arms with pinnules. The latter are given off in rows from 

 every joint at one side of the arm only — not alternately from opposite 

 sides — from one axillary to another, reversing their position in the succes- 

 sive divisions. The Hybocrinidae, which represent one of the most primi- 

 tive forms of the Fistulata, possess but one arm to the ray, and have neither 

 pinnules nor armlets. The Catillocrinidas have numerous simple, pinnuleless 

 arms, given off directly from the radials, w^ithout the interposition of 

 axillaries. 



In the Camerata the lower brachials take part in the calyx. That these 

 plates were free in the early larva, and were gradually incorporated, is 

 clearly indicated by the ontogeny of recent Crinoids, as well as the phy- 

 logeny of fossil ones. In the early Pentacrinoid larva of Antedon the 

 arms are free from the radials up, and the costals and distichals are sub- 

 sequently drawn into the calyx by the gradually increasing perisome. This 

 was evidently the case in the Camerata also. But while in the former the 

 perisome consists of soft tissues encrusted with limestone particles, the peri- 

 some of the latter is formed of well-defined plates. Our knowledge of the 

 ontogeny of Camerate Crinoids is of course limited; all we know is that 

 smaller specimens have a less number of interbrachial plates, that the number 



