EKaternal Structure of the Paleozoic Crinotds. 459 
species in which a subcentral proboscis is placed between the 
two small plates, and the whole vault looks like an immense 
proboscis. In these forms the four large plates, together 
with the two smaller ones, are pushed towards the anterior 
side of the specimen, while the centre plate rests with one side 
against the proboscis. 
There are other summit-plates following a radial direction, 
which are either attached to the apical pieces or separated from 
them by a belt of small polygonal plates. Their number, 
which varies greatly in different species, depends upon the 
number of primary arms that spring out directly from the 
body, no matter how often the arms branch afterward. In 
species with only two arms to the ray, each ray has two rows 
of corresponding plates in the dome: one large bifurcating 
plate forms the upper row, three plates the second row; two 
of the latter are brachial plates, the third one is an inter- 
brachial plate separating the two arms. In rays with three 
arms, there are eight plates in three series. The upper series 
consists of one large bifureating plate, which evidently corre- 
sponds with the third radial of the dorsal side. The second 
series, corresponding to the secondary radials, is composed of 
two plates, the plate towards the division with two arms being 
as large as the plate of the upper series, and the one towards 
the single arm much smaller. The third series is formed by 
three brachial and two interbrachial plates. In species with 
four arms to the ray, both radial pieces of the second series 
are large, and from each of them there originate two brachial 
pieces. As a general rule the summit-plates increase in pro- 
portion to the number of primary arms of a species in the 
same manner and on the same principle as the plates of the 
dorsal side. very radial from the third radial upward has a 
corresponding plate on the ventral side, and additional in- 
terbrachial plates between corresponding brachial plates above 
the arms. ‘Therefore in adult specimens, with some little 
practice, the number of arms can be ascertained nearly as well 
from the dome as from the dorsal side. The number of vault- 
pieces is enormous in some genera, especially if the radials 
branch off alternately, as, for instance, in Stérotocrinus, where 
some species have 120 to 180 arms. In looking at a full- 
grown specimen, with its many hundred apparently irregu- 
larly arranged vault-pieces, one would scarcely expect to be 
able to discover that this construction, in nearly all the Palzo- 
zoic Crinoids, is based upon a definite plan, and that plan the 
same as prevails below the arms. That this is the case may 
be successfully demonstrated in the young Strotocrinus, which 
has comparatively fewer summit-plates. 
31 
