84 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



So there is an almost uninterrupted series of forms from the Inadunata to 

 the most completely developed Camerata. 



It has been stated that in Dichocrinus the various orders of brachials, to 

 the last bifurcation, consist of two plates each, and that the plates of each 

 order form a syzygy, the epizygal bearing an arm instead of a pinnule. A 

 similar structure is found in most species of Platycrinus from the costals up, 

 and although the union between these plates may have been less close than 

 in Dichocrinus^ they evidently form a syzygy, except in cases in which the 

 first plate of the order is pinnule-bearing, as in P. Huntsvillce and P. Sarce. 

 It is very significant that in both these species, and a few others, the first 

 pinnule is given off from the first distichal, and the second on the same side 

 from the first palmar, thus showing that the arm partakes of the alter- 

 nation of the pinnules ; and this suggests the question w^hether all arm- 

 branches are not enlarged pinnules. In Eucladocrinus, which is actually a 

 highly diferentiated Platycrinus, the branches are given off alternately from 

 every second joint up to the end of the rays, exactly like the pinnules in 

 cases of syzygy ; but while in Platycrinus the axillaries are in their normal 

 condition, — the superior faces equally divided, — in Eiicladocrinus they are of 

 irregular form. They resemble enlarged pinnule-bearing plates, of which 

 the side supporting the next order of brachials is much wider than that 

 giving off the arm (Plate LXXIY). 



There can be but little doubt that in Eiicladocrinus the lateral arms in the 

 young Crinoid were pinnules ; and there is abundant proof that this was 

 also the case with the arms of other groups, as is w^ell shown by Glyptocrinus 

 Dyeri. In most species of Glyptocrinus, for example, G. decadactyluSy* the 

 second bifurcation takes place from the second distichal. In G. Di/eri, how- 

 ever, this plate gives off from one side in place of an arm a large pinnule, 

 more than twice the size of an ordinary one ; and a second pinnule, but 

 little smaller, starts off from the fourth distichal on the opposite side. Both 

 pinnule-bearing joints have nearly the shape of true axillaries, and what is 

 most remarkable, the arm bends outward, forming an angle, as if a true bifur- 

 cation took place. (Place XX., Fig. 1 a, h, c). The four or ^yq proximal 

 pinnules of this species are incorporated into the calyx, and it is quite evi- 

 dent that the growth of the armlets — or pinnules, whichever they are — ■ 

 was arrested by the rapid upward growth of the perisome. All succeeding 

 pinnules are sm.all, and given off' alternately from successive joints. 



* This species has twenty arms, and not ten as indicated by the name. 



