336 REPORT ON THE STATE CABINET. 



In the third volume of the Palceontologi/ of Neio York, page 134, 1 have 

 adopted this generic designation, applying it to a fossil from the Lower 

 Helderberg group of rocks, which hold nearly the same geological posi- 

 tion as the Ludlow rocks of England, in which the original of the genus 

 was found. The American species has a circular disc, composed of 

 squamiform spiniferous plates and five long flexuous rays. These rays I 

 have represented as composed, on the lower side, of a double range of 

 plates, as described and represented by Prof Forbes; but finding outside 

 of these a range of small ossicles to which are attached the spine-bases, 

 these have been shown as a part of an articulating spine (in the illustra- 

 tion, Plate vii, a, loc. cit.), an unnatural representation, which I am now 

 able to correct. 



In the species from the Lower Helderberg group, Protaster forhesi, the 

 ventral surfaces of the rays are composed of an ambulacral and adamlju- 

 lacral series of plates on each side. The ambulacral plates are obliquely 

 quadrangular and alternating in a slight degree ; the adambulacral plates 

 as seen from the lower side are narrow, elongate, oblique, and laterally 

 imbricating, presenting the appearance of an oblique ridge with the 

 anterior extremity projecting, and forming the point of attachment for 

 the spines, with which each one is furnished. When the ray is abruptly 

 curved, these plates project outwards, sometimes almost rectangularly ; 

 and when at the same time the ambulacral area is obscured by adhering 

 matrix, these plates might readily be mistaken for appendages of the 

 inner ranges. The pores are comparatively large, truncating the outer 

 adjacent angles of the ambulacral plates, while the base of one adambu- 

 lacral plate and the side of another form the exterior margin. The 

 centres of the upper sides of the rays are composed of two ranges of 

 subimbricating plates, which are closely joined along the median line ; 

 the marginal plates are the upper edges of the adambulacral plates, which 

 bear on their anterior ends one, two or three short spines. 



The structure of the lower side of the ray does not agree with the 

 description or with the figure given by Prof Forbes (loc. cit.), nor with 

 that of Mr. Salter, given as an illustration of Protaster miltoni* In the 

 latter species, Mr. Salter says the arms " are made up of a double row 

 of about forty pairs of squarish concave plates above, placed exactly 

 opposite, not alternating as in other species " (Plate ix, fig. 4 b). On the 

 lower side the ray is represented as made up of two ranges of plates, 



* Jlnnals and Magazine of Nat. History, second series, p. 330, Plate ix, flg. 4 c. 



