190 PALEONTOLOGY OF NEW-YORK. 



This genus is constituted to receive some very peculiar crinoidal remains, which, 

 till within a short period, were not fully understood. In the arrangement and en- 

 graving of Plate V in 1855, the figures 8 to 12 inclusive express all that was then 

 known regarding these fossils ; which had been observed as hemispheric or sub- 

 conical bodies with entirely smooth surfaces, giving no evidence of the point of 

 attachment for a column, and having the margins adhering to the imbedding stone, 

 or the cavity filled. On carefully freeing several of these from the stone, the forms 

 and character presented in the figures became apparent. In fig. 12, the cavity ap- 

 pears as if closed above by a solid concave plate; but this probably results from a 

 thickening of the walls, or from the thickening of the exterior wall alone, the 

 concave plate representing the base of the interior original cavity. The depressions 

 were evidently intended for the reception of other plates, which at that time were 

 entirely unknown. 



Recently I have obtained from Mr. Andrews of Cumberland, Maryland, a series 

 of specimens whicli fully illustrate the characters of the genus so far as regards the 

 plates of the body and arms, and also prove the mode of growth. By reference to 

 Plate Lxxxvn ( Crinoidese of the Oriskany sandstone), it will be seen from several 

 examples that these crinoids are sessile in the you)ig state, adhering singly or in 

 groups to other substances until fully developed, when they are separated from the 

 foreign bodies, and, gradually secreting calcareous matter to cover the cicatrix or 

 point of adhesion, become finally the smoothly rounded bases which we find so 

 numerously in the shaly limestone of the Lower Helderberg and the Oriskany 

 sandstone. 



In this progress of growth and separation, the base of the crinoid undergoes many 

 modifications of form; presenting itself with the angular outlines of the recently 

 attached surface, or with these angles removed, and the cicatrix in various stages 

 of obliteration, until finally the base becomes smooth and rounded, but varying 

 greatly in proportions of length and breadth. 



In the Oriskany sandstone, these bodies are equally as abundant as in the shaly 

 limestone of the Lower Helderberg; and they not unfrequently preserve the radial 

 plates at the base of the arms. 



