240 
PALAEONTOLOGY OF NEW-YORK. 
that the plates covering it were similar to those in the following species. The anteal or basal 
pectinated apertures present the characters noticed in the general review of these parts, having 
the lower one of the pair sub-triangular, with apex placed towards the base, while the higher 
one is nearly oval or sub-reniform. In the higher pairs, this relative position is reversed ; and 
in the pair placed nearly horizontal, the one slightly higher than the other is the triangular 
one. The triangular apertures have a thinner border, and are larger and less deep than the oval 
ones, which gradually expand outwards, and the border is labiate. The transverse bars which 
can be counted in the higher, or triangular ones of the left and right sides, and the base or anteal 
side, are respectively 9, 13 and 16 ; while in the lower or oval ones there are a less number, 
apparently having the same gradation, but not distinctly ascertained. If a line be drawn through 
the specimen in a direction from the anteal to the posteal side, the mouth and associated organs 
are on the left side ; the bases of two of the arms arch over these organs, including between 
them the pair of pectinated apertures on the left side : one of these arms is dichotomous below, 
or near the extremity. On the right side diverge three arms, two of which, arching over the 
right pectinated apertures, are dichotomous near their extremities; while one of the simple 
arms goes to the right of the anteal pectinated apertures, and to the left of the ovarian aperture. 
The bifurcation of the arms may not be of specific importance, but following or coincident 
with some other modifications. The arms are, as described, composed of a double series of 
plates laterally in contact, and resting in a shallow groove in the plates of the body. Each 
series of joints is similar, and composed of a large and small joint alternating with each 
other : the large joint has its inner margin convex, and longer than the outer margin ; the*-, 
smaller plate has a long external margin, while the inner margin is extremely narrow, and not 
projecting as far as the margins of the adjoining plates. The groove between the rows of the 
plates of the arms is excavated from the plates of both series, but not entirely through them, 
as they still join by their margins below. The sinus is made by the narrow margin or angle of 
the smaller plate, which is intercalated between each two of the larger ones, but does not pro¬ 
ject as far into the centre as those. At the bottom of this sinus, the tentacula take their origin. 
This groove, in the living condition of the animal, was undoubtedly occupied by some organ 
diverging from the centre, extending to the extremity of the arms, and finally diverging into 
the bottom of each sinus. From some remaining evidences, it seems that this organ was com¬ 
posed of minute articulating joints, probably covered by a fleshy envelope, and possessed the 
power of keeping up a communication from the centre along the line of the arms, -and to each 
individual tentaculum ; so that from the most remote points of these, sensation was communi¬ 
cated at once to the centre. The similarity of this groove to the canal or avenue on the under 
side of starfishes, is too obvious to escape attention. The little porous tubercle near the mouth 
also strongly reminds one of the madreporiform tubercle in Asterias and other Echinoderms. 
Fig. 1. The posterior side, showing the ovarian aperture destitute of plates closing it, the arms 
on each side arching over it, and the pectinated apertures on the right side, with 
the margin of those on the left, between which and the aperture is a dichotomous 
arm. 
