1 io 
PALAEONTOLOGY OF NEW-YORK. 
tlieless simple arms, or arms of a double series of plates, without armlets ; 
while in the following species a further modification takes place, affecting 
the structure of the bases of the arms, these appendages themselves 
preserving the structure of those in M. plumosus. Further discoveries may 
show some intermediate forms, by which these two extremes of develop¬ 
ment may be united. 
The differences here indicated are not dissimilar in value to those on 
which it is proposed to separate Ctenocrinus from Actinocrinus. The great 
ntimber of species under the latter genus renders such differences of more 
interest than in a genus composed of few species, like the Mariacrinus. 
Mariacrmus (Subgenus) Teclmocriims spinulosus (n.s.). 
Plate LXXXV. Fig. 1 - 18. 
Body somewhat spheroidal, slightly contracted at the junction of the arms. 
Basal plates four, small, wider than long; three pentagonal and one 
hexagonal, marked by strong radiating ridges. First radial plates about 
as long as wide ; three heptagonal, the others unknown. Second radial 
plates hexagonal. Third radial plates pentagonal. Interradial plates (on 
the side fully seen) four ; lower one regularly hexagonal. Brachial plates 
double; one resting on each sloping side of the third radial, and giving 
origin to a pair of arms. Surface of plates ornamented by ridges and 
nodes, or short spines. 
Arms composed at base of a single series of joints, becoming double above? 
Tentacula unknown. 
Column somewhat pentagonal, consisting of joints which are deeply striate 
on their articulating surfaces; each alternate joint furnished with nodes 
or short spines from the angles. 
This beautiful species is readily recognized from its general structure, and from 
its strongly ornamented surface. The column is obtusely five-angled; and from the 
deeply striate articulating surfaces, and nodose or spiniferous angles, may be re¬ 
cognized even in small fragments. The alternating joints are without nodes. 
The plates of the body, for three or more series above the base, are marked by a 
central spine or node, which is surrounded by a series of smaller nodes, often, or 
