External Structure of the Paleozote Crinoids. 461 
around the, apical and radial plates. The species of these 
genera, though comparatively of large size, have generally 
but two primary arms, and consequently for each ray but 
one radial dome-plate, which is here placed at some dis- 
tance from the arm-bases. In the adult Megistocrinus the 
radial as well as apical plates are extremely large, and stand 
forth conspicuously, and each one separately, among the sur- 
rounding minute polygonal pieces. In the young Megisto- 
crinus, however, and in the Rhodocrinide generally, the apical 
pieces and the radial plate are placed side by side, being sur- 
rounded by the polygonal plates. The form and size of the 
principal summit-plates, the distribution and number of the 
interradial pieces, afford most excellent characters for distin- 
guishing many genera. In Agaricocrnus all apical and radial 
pieces are large and tuberculous, the few interradials are small. 
In Dorycrinus the centre plate and first radials are spiniferous 
or nodose. In Amphoracrinus the four large apical pieces 
are spiniferous or tuberculous, the radials nodose. In Platy- 
crinus and Hexacrinus the apical plates are very prominent, 
often tuberculous, the radial portions are somewhat constructed 
like the rows of small alternating plates of the Cyathocrinide. 
In Batocrinus all summit-plates are nodose and almost of 
equal size. 
The apical plates can be distinguished in other groups as 
well as in this. They surmount the vault of Synbathocrinus 
and Cyathecrinus, cover the central opening of the Blastoids, 
and can be traced in many of the Cystideans. This, with the 
further fact that they are so largely developed in young 
specimens, that they cover and protect some of the most im- 
portant organs of the inner cavity, shows their great import- 
ance, and leads us to infer that they were the first solid parts 
developed on the ventral side in young Crinoids. The centre 
piece corresponds evidently with the basals of the dorsal side, 
the surrounding plates to the subradials (the two smaller plates 
separated by the anus forming together one large one), which, 
on the other hand, were undoubtedly the first-developed parts 
of the dorsal side, and the parts which are the most highly 
developed in the Cystideans. 
The above groups, representing the three principal plans 
upon which the vault is constructed, embrace, according to my 
views, not only all those Paleozoic genera which were sup- 
posed to be covered by a membranous surface, but nearly all 
Paleozoic Crinoids that are known. There are some few 
genera, as, for instance, Hucalyptocrinus, with a very peculiar 
superstructure at the ventral side, whose affinities I have not 
been able to determine. There is the genus Calceocrinus, 
