OWEN AND SHUMARD'S FOSSIL CRINOIDEA. 91 



plates unknown. Of the accessory pieces, one plate only remains ; this is nearly square, 

 and is placed between the two heptagonal plates of the second series. 



Column very small, consisting of round joints, with articulating surfaces striated 

 in radii. 



Dimensions. — Height of first series, 1 line ; of second series, 3.5 lines ; greatest 

 width of calyx, 5 lines ; diameter at base, 2 lines. 



From the imperfect condition of the only specimen we have seen of this species, 

 its characters cannot be well determined ; the description given above, however, will 

 enable any one to identify it without difficulty. 



It was found in the pentremital layers of the carboniferous limestone at Chester, 

 Illinois, associated with Pentremites Jiorealis, P. CheroTceus, Agassizocrinus 

 dactyliformisy and Productus Flemingii. 



EXPLANATION OF FIGURES. 



Fig. 3. a. Basal view. 

 " h. View of the side. 



POTERIOCRINUS SPINOSUS. (NeW Sp.) 



PI. XL fig. 4. 



Calyx. — Cup-shaped, small; plates smooth; mouth proboscidiform ; arms five; 

 subdivisions thirty, furnished with thorn-like projections at the points of bifurcation. 

 Basal plate pentagonal, small, slightly concave, with a minute pentagonal opening 

 in the centre ; divisions of the plate not perceptible. First series of plates pentagonal, 

 small, thin. They surround the basal plate, and form by their union a little pentagon, 

 flat or slightly concave, which is almost completely hidden from view by the last 

 joint of the column, so that, in order to obtain a good view of them, it is necessary to 

 remove this portion of the animal. The second series consists of five pieces, three 

 pentagonal and two hexagonal ; they are larger and more massive than the plates 

 beneath ; surface moderately convex, inferior margins bent inwards to articulate with 

 the plates of the first series. Superior or arm-hearing plates, wide, pentagonal, thick, 

 moderately convex ; facet for the articulation of the arms, wide, situated on the upper 

 edge of the plates, occupying their entire breadth. Accessory pieces three, irregular. 



The surface of the calyx exhibits a number of small indentations, one of which is 

 situated at each of the angles of the second series, superior, and accessory pieces. 



Main arms five, each composed of a single joint, which is irregular, elongated and 

 furnished with a thorn-like projection superiorly. The upper edge presents two 

 oblique facets, supporting two hands, consisting of wedge-shaped joints, arranged in 

 single longitudinal series. The number of joints in the different hands varies from 



