18 PKOr. p. M. DrNCAN's EEVISION OF THE 



Interradia one half as broad as the ambulacra, with five or 

 more vertical rows of overlapping plates, which diminish in 

 number towards the poles ; the plates imbricate from above down- 

 wards {prolahly the reverse) and from the central range outward. 



Surface covered with small granules for the articulation 

 of minute spines. Jaws consisting of large subtriangular, 

 truncated, conical pieces deeply furrowed towards the ends and 

 perforated in the central part. G-enital plates probably with four 

 pores. The spines minute and acicular. 



Fossil. Carboniferous : N. America. 



Genus Pholidocidaeis, Meek Sf Worthen, 1869, Proc. Acad. Sci. 

 PTiilad. p. 78 {under Lepidocentrus) ; 1873, Geol. Illinois 

 (I'al(Bont.), vol. V. p. 510, pi. xv. fig. 9. Loven, Ftudes, 

 1874, p. 40. Zittel, 1879, Falcsont. Bd. i. p. 482. 



Fragments belonging to individuals of from 90 to 100 milli- 

 metres in diameter. 



Interradia with five or more rows of plates imbricated aborally 

 and laterally, granular, thin, rounded, convex, unequal, those 

 nearest the ambulacra three or four times the size of the others, 

 elliptical, higher than broad, projecting. On one surface 

 (ventral?) there are primary tubercles one to a plate, placed 

 centrally, and perforated, and surrounded by two smooth rings ; 

 similar tubercles on the opposite surface of the test only on the 

 ambulacral plates. Spines subulate, finely striated longitudinally. 



Ambulacra broad, with six vertical rows of small plates, variably 

 shaped, oval, rhomboidal, or with the angles rounded; plates 

 imbricating adorally ; with a moderate-sized mamelon, and the 

 pores in single or sometimes double pairs, in a depression. 



There are also small spines and small buccal scales present. 



Fossil. Inferior Carboniferous : N. America. Type P. irre- 

 gularis, Meek and Worthen. 



There is a singular question about the zoological position of 

 PalcBodiscus, Salter, for the late Sir Wyville Thomson placed it 

 as a synonym of Fchinocystites. There are specimens in the 

 Museum of Practical Geology, Jermyn St., and in the British 

 Museum ; and it is tolerably evident that there are forms there 

 which simulate the typical Falceodiscus, which Salter decided to 

 be an Asteroid. Two of these forms in the British Museum are 

 associated, and properly so, with the Palseechinoidea, but are 

 flattened, badly preserved semicasts. The interradial parts have 



