254 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
obscure, flattened ridges, which grow more conspicuous on approaching 
the arms. Infrabasals small, rarely extending beyond the column, and 
resting, as a rule, within a concavity formed by the lower part of the 
basals. Basals heptagonal, their upper faces truncated. Radials in three 
of the rays pentagonal, in the two posterior ones frequently hexagonal. 
When there are no palmars the three to six lower distichals take part in the 
calyx, and the succeeding ones are free arm plates. Arms short, slender, 
branching; composed of two rows of cuneiform pieces, alternately arranged 
and interlocking. Interradial areas constructed of numerous plates; the 
plate between the radials, which is much larger than the others, is followed 
at the regular sides by two interbrachials in the second row, at the anal side 
by three, but there is no median ridge or continuous row of anal plates. 
The upper interbrachials connect imperceptibly with the plates of the disk. 
Interdistichals always represented. Ventral disk, as observed in A. desideratus, 
composed of small irregular pieces, which close over the ambulacra, except 
near the outer margin of the integument, where their covering pieces are 
exposed. Column round, the edges of the nodal joints largely projecting 
over the internodal ones; axial canal large, pentalobate. 
Distribution. — Probably restricted to the Trenton group of America. 
Type of the genus: Archeocrinus lacunosus (Billings). 
Remarks. — Archeocrinus has close affinities with Rhodocrinus, and it is 
somewhat difficult to point out the structural differences. The calyx of the 
former is relatively larger, the arms shorter, and it has but two interbrach- 
ials in the second row; while Rhodocrinus, as a rule, has three plates in the 
second and all succeeding rows; and the anal interradius very often has the 
same arrangement of plates as the others. 
Glyptocrinus marginatus Billings, which in 1881 we placed under Archeo- 
erinus, proves to be a monocyclic form, and will have to be referred back to 
Glyptocrinus as a somewhat aberrant type. Whether Rhodocrinus asperatus 
Billings belongs here, cannot be ascertained from the imperfect state of the 
specimen. Lyriocrinus sculptus S, A. Miller (Archeocrinus sculptus W. & Sp.) 
is a synonym of Rhodocrinus vesperalis White, which has been placed under 
the genus Diabolocrinus. 
