334 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
berances, which represent ten longitudinal ridges upon the outer surface. 
The ridges meet the upward prolongations of the plates of the first ring, 
with which they are suturally connected and form a solid wall; while the 
alternate grooves form the inner part of the niches. The plates of the third 
ring are narrower than those of the second, and like them provided with ten 
longitudinal elevations, which are overlapped by certain projections passing 
down from the plates of the fourth rmg. The plates of the upper ring are 
quite remarkable as forming the upper part of the anal tube, the top of the 
crown, and at the same time the encasement for the tips of the arms. They 
are constructed upon a plan similar to that of the large plates of the first 
ring, and, like them, have wing-like extensions, which from a ventral aspect 
present a well defined decagonal star, with a vacant space at the summit. 
The lower ends of these wings meet the upper ends of the wings of the first 
ring, so as to form, together with the projecting surfaces upon the plates of 
the two middle rings, ten continuous walls, which extend from the edges of 
the dorsal cup to the top of the crown. The open space between them 
represents the end of the anal canal, which is closed variously by from five 
to ten small irregular pieces surrounding the anal opening, and sometimes by 
additional larger plates. The anus in some species is drawn out to a tube 
of great length, extending far beyond the limits of the arms, but more fre- 
quently rises but little above the top of the compartments. Arms heavy, 
arranged in pairs, each pair occupying one of the ten compartments, their 
backs almost even with the edges of the partitions, their lateral faces abut- 
ting against the sides. They have a wide ventral furrow, and are composed 
from the third or fourth plate up of two rows of short, transverse pieces, 
which are so closely united by suture, that the arms must have moved en 
masse upon the calyx. Pinnules long and closely folded. The visceral 
cavity is formed by the plates of the dorsal cup and the two lower rings of 
plates of the disk; the plates of the two upper rings, which form the neck- 
like prolongation, being, properly speaking, plates of the anal tube. The 
ambulaera, on entering the calyx, follow the grooves at the inner floor, and 
meet near the top of the second ring. Column moderately large, generally 
round; composed of rather long joints with pentapetalous axial canal. It 
has no lateral cirri, but branches at the end into hundreds of little rootlets. 
Distribution. —A leading form of the Upper Silurian, and well repre- 
sented in this country as well as in Europe. A single species is known from 
the Devonian: Eucalyptocrinus rosaceus from the Eifel of Germany, the type 
of the genus. 
