LOWER HELDERBERG ROCKS. 117 



Abms bifurcating upon each 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 

 plates, 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 striae. 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 arras of the specimen are not 

 entire, the portion remaining being more than four times as long as the body. 



Another specimen of what apj>ears 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 tie 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. 



