Williams.] 84 (April 20, 
A few detached calyx plates with similar surface markings, but propor- 
tionatelyfonger and narrower than those of A. Jthacensis, were found in 
another exposure of the rocks near Ithaca. 
The tubercles were few and scattered, this may represent another spe- 
cies. The generic characters of this new type of Crinoid may be defined 
as follows : 
Arthroacantha, nov. gen. 
(From ’¢p0pov, articulation and ’¢éxeavda spine.) Calyx obconical or 
broadly cup-shaped ; height about equal to the breadth. Basal disc broad, 
shallow, hexagonal, composed of three subequal plates, 
Following the basal disc are six large subequal plates, five of which are 
primary radials, and the sixth is the anal. 
The first radials are slightly higher than broad with gently diverging 
sides, the upper margin excavated by a deep covered notch occupying 
about one third the total width of the upper edge. 
In this notch lie the second radials, small and short plates which arch 
outward and continue upward the rounded carination that begins on the 
upper part of the first radial. 
The third radial is triangular, smaller than the second and supports the 
first plates of the free arms which start out from the radial at a broad angle. 
The arms aré ten, and, in the typical species are several times as long as 
the height of the calyx, and bifurcate at least twice, and broadly diverge 
at each branching. 
They are composed of plates which are narrowly wedge-shaped at the 
base of the arm, the first two or three reach across the breadth of the arm, 
but seriatim they become shorter, the wedge points more blunt, and the 
outer portion of the margin more nearly parallel, and for the main part 
of the arm the plates interlock along the median line, forming a zigzag su- 
ture, the points of the plates from each side reaching less than two-thirds 
across the surface of the arm. 
Each arm plate bears a slender pinnule of five or more joints. 
The anals are a little narrower than the first radials, and have less 
diverging sides.’ 
The vault is composed of numerous small plates, and was probably low 
and arching. 
The surface of the calyx plates is beset with low scattered, rounded tu- 
bercles, pitted at the apex. 
The same tubercles are seen on the plates of the vault, a single tubercle 
for each plate. 
Upon the vault there are five narrow spaces, without tubercles, radiating 
from the center; they consist of two rows of interlocking plates which 
were probably thinner than the spine-bearing plates; all the intervening 
plates have tubercles. 
Along the upper rim of the calyx is a row of small plates which lack 
the tubercles ; also, the tubercles are wanting on the second and third 
