68 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
between Merocrinus and Docrinus, and between Oltawacrinus and Heterocrinus, — 
dicyclic and monocyclic forms respectively. 
While the evidence thus points to the probability that the one group was 
derived from the other, it is difficult to explain the change in the orientation 
of the stem. That the whole crown passed through a revolution of 36° is 
highly improbable. The change probably took place in the body of the 
stem, modifying its form to suit the condition of the base. The stem is 
round in most of the earlier Crinoids; but we find occasionally within the 
same family quadrangular stems from a quadripartite base, together with 
pentangular stems from a quinquepartite base, which goes to prove that the 
stem, to some extent at least, adapted its form to the conditions of the base. 
In a similar manner interradial stems may have changed into round stems, 
and these into radial stems. This, however, does not explain the change 
in the orientation of the canal, and that of the cirri. The latter are 
directed to the angles of the axial canal, through which they communi- 
cate with the chambered organ. 
The only writer, besides Bather, who has discussed the derivation of the 
two groups is Dr. J. Walther,* who takes it for granted that the simpler — 
monocyclic —form is the ancestral one. His arguments were suggested from 
his study of the pentacrinoid larva of Antedon, and are extremely hypo- 
thetical. They are based upon his peculiar interpretation of the basals and 
infrabasals, which, as he now will himself admit, cannot be upheld since Bury 
discovered infrabasals in the larva of Anfedon. 
C. The Radials. 
The term “radials” is applied by us only to the first plate of each ray. 
All succeeding plates in a radial direction, whether free or incorporated into 
the calyx, are brachials. The name was given by Miiller to all plates up 
to, and including the first axillary, and all pieces beyond were regarded by 
him as armplates. The same view was taken by Roemer, de Koninck, and 
other European authors. The American Palxontologists applied the term 
not only to the plates of the first order, but to all plates of the rays that take 
part in the calyx. They distinguished those of different orders as primary, 
secondary, and tertiary radials, according to rank. 
* “Untersuchungen tiber den Bau der Crinoideen.” Palwontographia, 1866, Bd. XXXIT., pp. 180- 
199. 
