30 PEOF. P. M. Duncan's bevision of the 



Grenus Pbeischodomtjs, McCoy, 1849, Ann. Sf Mag. Nat. Hist. 

 ser. 2, vol. iii. p. 251. W. Keeping, 1875, Quart. Journ. 

 Geol. Soc. 1876, vol. xxxii. p. 35, pi. iii. figs. 1-5. Worthen Sf 

 Miller, 1883, Geol. SfPal. Illinois, vol. vii. p. 333. {Amended.) 



Syn. Perischocidaris, JN^eum. 



Test spheroidal, depressed, subpeutagonal in outline. 



Apical system central, with five broadly pentagonal basal plates 

 surrounding a small periproct, each with from 6-8 genital per- 

 forations ; radial plates small ; anal plates exist. 



Ambulacra narrow, straight, sunken, overlapped on either side 

 by the interradia ; plates in two vertical series, numerous, small, 

 low, broad, and either regular in shape, elongate pentagonal, or 

 wedge-shaped, the small end of one plate in contact with the large 

 part of its neighbour ; plates overlap from the apex actinally, 

 each with a pair of pores in simple vertical series or slightly al- 

 ternating ; surface minutely granular. 



Interradia broad, with five vertical rows of large scale-like plates 

 at the ambitus, diminishing in number to two or three at the apex. 

 Plates variable in thickness, thick or thin, convex and irregular in 

 outline, those of the middle rows the most symmetrical, trapezoidal 

 or depressed hexagonal ; plates of the middle row overlap those 

 of the row on either side and these the other rows to the ambu- 

 lacra ; each plate also overlaps, with its aboral edge, the plate 

 situated apically to it ; the highest plates overlap the basal plates 

 of the apical system. 



Ornamentation granular, homogeneous, and of small and also a 

 few large, perforate, non-crenulate tubercles with a low, depressed, 

 broad, conical boss, placed upon a circular scrobicule, A larger 

 tubercle upon some interradial plates close to, or in the second 

 row from the ambulacra, or more than one, often with a circle of 

 the smaller kind near the edge of a plate. Jaws large ; teeth 

 large and grooved. Spines, some small, aciculate, and striated ; 

 others larger but still short, smooth and cylindrical, tapering, 

 broadest inferiorly, without a ring or collar, striated. 



Fossil. Carboniferous : Wexford, Ireland, and Clitheroe, Eng- 

 land ; Scotland ; Europe. 



(The diagram given by Keeping, op. cit. fig. 3, should be 

 reversed.) 



Tlie type specimen of P. biserialis, McCoy, in the Wood- 

 wardian Museum, Cambridge, has a great adoral underlap of the 

 interradial plates. 



