PALiEOZOlC FOSSILS. 293 



GENUS PROTASTER (Forbes). 



Protaster : Fokbes, Memoirs of the Geological Survey of Great Britain, Decade i. 



1849. 



The description of the genus is as follows : 



*' Body circular, covered with squamiform plates; genital openings in the 



*' angles of junction of the arms beneath : arms simple, formed of alter- 



" nating ossicula." 



In the third volume of the Palaeontology of New- York, page 134, I 

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

 Lower Helderberg group of rocks, which hold nearly the same geological 

 position 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 adambu- 

 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 ante- 

 rior 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 ambu- 

 lacral plates ; while the base of one adambulacral 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 descip- 

 tion, 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 



