210 Twenty-fourth Report on the State Museum. 



Genus — HETEEOCEINUS Hall, 



This genus, originally described in the first volume of the Palaeon- 

 tology of New York, is there represented as being destitute of basal 

 plates. This is true of many specimens examined, and of most of the 

 species in their ordinary condition. Some of them, however, do pre- 

 sent small points or nodes at the summit of the column, alternating 

 with the subradial (described as basal) plates; and these should 

 undoubtedly be regarded as basal plates. Other specimens show the 

 existence of minute basal plates on the removal of the column, 

 proving that these parts of the calyx exist in an undeveloped 

 condition. 



Were the basal plates developed, the structure of the calyx would 

 be the same as in Potekiocrinus ; and in the absence of these plates, 

 those which are the subradials in that genus become the basal or 

 lower series, resting directly upon the column in the forms referred 

 to Heteeooeinus. Since we have acquired farther knowledge of the 

 structure and mode of growth in Crinoidea, we have become 

 acquainted with similar conditions in other genera, particularly of 

 Lower and Upper Silurian forms. It may become a question of some 

 interest, whether species or genera, where the absence of certain 

 parts may be presumed due to non-development, or want of external 

 development, shall be entitled to rank as genera on that account. 



Heteeooeinus consteictijs n. sp. 

 Plate 5, figs. 13, 14. 



Body and arms somewhat above the medium size, robust, expand- 

 ing gradually from the basal plates to the top of the second radials 

 of the principal rays, and more abruptly from below the origin of 

 the arms, giving a constricted appearance to the fossil at that point. 

 Basal plates very low, not more than half as high as the width. 



In three of the rays — the anterior, left antero-lateral and right 

 postero-lateral — the bifurcation takes place on the third plate ; the 

 first and second plates are quadrangular, and of nearly equal height and 

 width; the third is short pentangular, and directed outward from the 

 plane of the plates below. In the right antero-lateral ray, the bifur- 

 cation takes place on the fourth radial plate, the constriction at the 

 top of the third, and the plates broader than high. In the left postero- 

 lateral ray the bifurcation is on the fifth radial, and the constriction 

 still at the top of the third, the fourth plate being very short. Arms 

 above the bifurcation strong, composed of a single row of wedge- 

 form plates, every third one of which, on alternate sides, or every 

 sixth one on the same side, bears a short, thick armlet, which is 

 composed of quadrangular plates. 



