188 C. Wachsmuth—Structure of Paleozoic Crinoids. 
nus. Here, the basals, primary radials, first anal and first 
interradial pieces are comparatively large, while the higher 
series of interradials are yet absent or but slightly developed. 
The radials of the higher orders, which in adult specimens 
form a part of the body, are in young specimens free arm 
plates, unsupported by any interradial or interaxillary pieces. 
The 8, sberlin which spring directly from the body in 
adult specimens, in the young branch alternately right and left 
after emerging from the y, the spaces between the bases of 
the branches being subsequently filled by the upward growth 
of the body, so that the branching, instead of occurring in the 
arms, seems to be. completed in the body walls. So, for 
instance, the young Strotecrinus umbrosus has at first but four 
arm openings to the ray, ata later period it is found to have 
eight, and m the adult state twelve, being a separate opening 
for each arm. 
The rule, that the number of summit plates increases in pro- 
portion to the number of primary arms, holds good with refer- 
ence to the young specimen. The young Strotocrinus has fewer 
plates than the adult individual (the difference being in pro- 
portion to the state of growth), and these are arranged in the 
same order, and are as easily recognized as those of the sim- 
lest species of this group. The apical and principal radial 
pieces are larger than the intervening interradial plates which, 
exceptionally in this genus, attain by age the same size as the 
apical and radial pieces. The interradial plates of the vault oc- 
cupy the intermediate spaces between the radial areas. As 
their number depends greatly upon the age of the individual, 
they vary often im the same species. In species with but lew 
arms, we find comparatively few interradials, and those are gen- 
erally smaller than the other plates. The latter is especially 
true in young specimens, as also in small species. Sometimes, 
generically from an adult Actinoerinus idialis, and both have the same 
entation with the same number of arm openings, they but 
slightly in specific cha Act p is the ical species 0: 
of all Strotocrini, which idea seems to be r confirmed by the geological suc- 
cession. The former group occurs only in the Lower, and Strotocrinus only in the 
Upper Burlington limestone. 
