CONTRIBUTIONS TO PALiEONTOLOGY. 131 



a greater number of plates : the first one equals in size the first 

 radial, and supports three in the second range, with five in the 

 third range; above which, the arrangement cannot be well de- 

 termined. 

 Surface of plates marked by radiating ridges, which form, with 

 those from adjoining plates, a series of isosceles triangles : in the 

 lower part of the cup they are a triple series, and in the upper 

 part a single series. The ridges which traverse the ray are stronger, 

 and form a sharp carina, with strong nodes on the second radial 

 plates; while on the centres of all the other plates are low an- 

 gular nodes. 



This species differs from A. nyssa in the form of the cup, in the surface 

 characters, and in the divisions of the rays, which take place lower down 

 in the calyx. It bears considerable resemblance to A. tenuis of De Ko- 

 NiNCK {Monograph^ pa. 128, pi. ii, f. 3). 



Geological formation and locality. In the shales of the Hamilton group: 

 Western New- York, C. A. Write, collector, 



ACTINOCRINUS PRJilCURSOR (n.s.). 



Body small, short, subglobose, truncate at the base. Basal plates 

 small, depressed below for the reception of the column. First 

 radial plates proportionally large, much wider than high. Second 

 radials less than half the size of the first, quadrangular. Third 

 radials very small, subtriangular, with the lateral angles scarcely 

 truncate; the upper sloping margins supporting an arm on one 

 side, and on the other two supraradials, the upper one of which 

 supports two arms; giving three arms to each ray. 



Interradial areas consisting of three plates each ; the first of mo- 

 derate size, supporting two small ones in the second range, above 

 which the plates more properly belong to the dome. Anal plates 

 ten or more; the first little more than half as large as the first 

 radial plates, supporting three in the second series and five in the 

 third, with small plates above, uniting with those of the dome. 

 Arms strong, composed of a double series of short plates, each of 

 which has an elevated ridge in the middle, giving a very rugose 

 structure to the arm. 



Surface of body-plates elevated, and roughened by confluent gra- 

 nulse. The dome is imbedded in rock, and not determined. 



This species is of the type of A unicornis of the Burlington limestone 

 (Carboniferous), and strongly resembles it in the calyx and arms; differ- 

 ing in some details of structure, and in surface characters. 



