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. Huntsville and P. Sare. 
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 whether all arm- 
branches are not enlarged pinnules. In LZucladocrinus, which is actually a 
highly differentiated 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 Playcrinus the axillaries are in their normal 
condition, — the superior faces equally divided,—in Lucladocrinus 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 LX XIV). 
There can be but little doubt that in Zucladocrinus 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 well shown by Glyptocrinus 
Dyeri. In most species of Glyptocrinus, for example, G. decadactylus,* the 
second bifurcation takes place from the second distichal. In G@. Dyeri, 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. la, d,¢). The four or five 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 small, and given off alternately from successive joints. 
* This species has twenty arms, and not ten as indicated by the name. 
