460. Mr. C. Wachsmuth ox the Internal and 
The young specimen, in genera with numerous arms, has 
fewer arm-openings than the adult, though both have the 
same number of arms. This is best observed in the young 
Strotocrinus. 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 de- 
veloped. 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 interaxil- 
lary pieces. The arms, therefore, which spring directly from 
the body in adult specimens, in the young branch alternately 
right and left after emerging from the body, 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 free arms, seems to be completed in the body- 
walls. So, for stance, the young Strotocrinus umbrosus has 
at first but four arm-openings to the ray ; at a later period it 
is found to have eight, and in the adult state twelve, being a 
separate opening for each arm*. 
The rule that the number of summit-plates increases in 
proportion to the number of primary arms holds good with 
reference to the young specimen. The young Strotocrinus 
has fewer plates than the adult individual (the difference being 
in proportion to the state of growth); and these are arranged 
in the same order, and are as easily recognized, as those of the 
simplest 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 occupy the intermediate spaces between the radial areas. 
As their number depends greatly upon the age of the indivi- 
dual, they vary often in the same species. In species with 
but few arms, we find comparatively few interradials, and 
those are generally smaller than the other plates. The latter 
is especially. true in young specimens, as also in small species. 
Sometimes (as, for instance, in some Megistocriné and all Rho- 
docrinide) the greater part of the summit is covered by large 
numbers of small polygonal plates which form regular belts 
* A young Strotocrinus, unless the arms are attached, cannot be distin- 
guished generically from an adult Actinocrinus proboscidialis; and as both 
have the same peculiar ornamentation with the same number of arm-open- 
ings, they differ but slightly in specific characters. Act¢nocrinus probosci- 
dials is the typical species of a small group of beautifully ornamented 
Crinoids, and it is evidently the forefather of all Strotocrinz, which idea 
seems to be further confirmed by the geological succession. The former 
group occurs only in the Lower, and Strotecrinus only in the Upper Bur- 
lington Limestone. 
