NIAGARA GROUP. 



187 



Genus GLYPTASTER( nov. ge?i.). 

 [ Gr. yXuit-os, sculptilis, and atfrJp, stella.'\ 



Pelvic plates five, marked by strong diverging ridges whicli unite on the plates of the 

 second series, forming five strong ridges, which again bifurcate, sending one division to 

 the base of each arm. Structure of upper part of cup unknown ; arms ten, composed of 

 a double series of plates. 



This species is closely allied to Glyptocrinus ill the structure of the calyx, which can 

 not be fully determined. 



581. 1. GLYPTASTER ERACHIATUS (n. 5p.). 



Pl. XLI. Fig. 4 a, b. 



Structure of the calyx not ascertained ; surfiice granulate or tuberculated, in addition 

 to the strong ridges ; arms ten, nearly cylindrical and much elongated. 



The specimen figured is tlie only one which has fallen under observation, and the 

 shaly matter adheres so strongly to the plates as to render it impossible to ascertain the 

 true structure. The arms are spread out on the surface of the stone, and the base of the 

 specimen turned upward so that the junction of the arms with the plates below can not 

 be seen. 



Fi*'. 4 a. The specimen as it appears on the surface of a slab of shale. 



Fig. 4 6. A part of one of the arms enlarged, showing more distinctly the arrangement of the plates 

 of the arms. 



Position and locality. In the shale at Lockport. 



In the Lower Silurian period, there is a single species in the Chazy limestone, marked 

 by strong ridges diverging from the centre to the margins of the plate ; and another in 

 the Hudson-river group, and the Blue limestone of the same age at the west. There is 

 probably a third species, and possibly a fourth, in the Clinton group ; judging from the 

 fragments of arms and columns found in that position, and figured on Plate A 41. These 

 three, with the one figured above, are the only ones presenting these external characters 

 at present known to me in the whole range of the New- York strata. Were it not that 

 similar characters are shown (according to figures) in species of Actinocrinus from the 

 Carboniferous period, I should be inclined to regard this marking as characteristic of 

 Silurian crinoids. 



