LOWER 1IELDERBERG ROCKS. 
117 
Arms bifurcating upon eacb of the first brachial plates; the first four or 
five articulations of each being simple plates of an unequal quadrangu¬ 
lar form, and gradually passing into a double series of interlocking 
wedgeform or somewhat hexagonal plates, which have their outer and 
upper angles thickened and obliquely truncated for the attachment of 
the strong tentacles. Tentacula composed of long joints, which are 
thickened at their bases and articulating extremities. Interbrachial 
plates heptagonal; the two lower sides resting on the adjoining radial 
jfiates, and laterally upon each side against the first and second brachial 
plates. 
Proboscis unknown. 
Column round; in the small portion attached to the specimen, consisting 
of alternating thicker and thinner articulations. 
This beautiful and well-marked Platycrinus presents all the characteristic features 
of the genus as seen in the Carboniferous species. In all its external characters, it 
differs from either of the species just described : the basal plates are proportionally 
smaller; and these, with the first radials, are strongly marked by radiating ridges 
or strife. The upper edge of the first radial is not deeply excavated, as in those, but 
is broadly and slightly concave; and the second radial is a low triangular plate 
with a long base. The second radial supports two instead of one brachial plate; and 
upon each of these the arm-plates bifurcate, giving origin to four arms from each 
ray, or twenty altogether. The joints of the arms are very prominent externally, and 
the thickening of the upper truncated angle gives them a peculiar appearance. The 
tentacula are remarkable in the thickened articulating extremities; in which re¬ 
spect they differ not only from the preceding species, but from all the carboniferous 
forms that have fallen under my observation. The arms of the specimen are not 
entire, the portion remaining being more than four times as long as the body. 
Another specimen of what appears to be the same species, enclosed in the solid 
stone and broken through the middle, shows a long slender proboscis. 
Fig. 1. The individual represented of the natural size. 
Fig. 2. Enlargement of arms and tentacles from the second radial plate ; the two contiguous 
arms of the two pairs being continued with the tentacles as far as seen in the 
specimen. 
Fig. 3. Enlargement of the plates from the basal through the radial and brachial plates, 
and continuation of a single arm. 
Fig. 4. Diagram showing the structure. 
Geological 'position and locality. In the calcareous layers of the shaly limestone 
of the Lower Helderberg group, Schoharie. 
