14 PEor. p. M. Duncan's eevision of the 



lacrum ; the outer pairs either in demi-plates or in primaries, 

 the inner pairs always in primaries, which may, however, be short ; 

 th.e outer row is associated with the plates, which form decided 

 salient angles at the ambulacro-interradial sutures ; pores of 

 pairs separated by distinct septa and without peripodia, always 

 distant from sutures. Ornamentation of ambulacra of small 

 granules and one or more transverse rows of a few very small 

 tubercles consisting of a flat scrobicule and a small boss. 



Interradia broad, convex, with from five to eight vertical rows 

 of thick tumid plates diminishing in number towards the poles ; 

 middle plates hexagonal, the adambulacral pentagonal and with 

 the ambulacral edge with salient and reentering angles to fit the 

 corresponding structures of the ambulacral plates ; some obliquity 

 of the edges of all the plates, but no true overlap. Ornamentation 

 of numerous small, close tubercles with a flat scrobicule and a 

 boss ; there may be a linear ornamentation also ; spines small, 

 acicular, short. Jaws and teeth with a groove. 



Fossil. Upper Silurian: England. Carboniferous Limestone : 

 England, Ireland, Scotland, Europe; N. America. 



The reasons for restricting the species of Palcsechinus to those 

 with two vertical rows of pairs of pores on each side of an ambu- 

 lacrum and for insisting upon the presence of radial plates have 

 been considered in a late publication (Ann. & Mag. Nat. Hist. 

 ser. 6, vol. iii. p. 196, 1889), and the necessity for enlarging the 

 interesting genus Rhoechinus, W. Keeping, has been shown in 

 the same place. The genus BhoecMnus how admits the forms 

 of Palgeechini with only a single vertical row of pairs of pores on 

 each side of an ambulacrum, such as P. elegans, McCoy. 



Genus B;Hoechinus, W. Keeping, 1875, Quart. Journ. Geol. Soc. 

 vol. xxxii. (for 1876), p. 37. Duncan, 1889, Ann. Sf Mag. 

 Nat. Hist. ser. 6, vol. iii. p. 205. (Amended.) 



Test small or moderate, spheroidal. 



Apical system with five basal plates with several perforations ; 

 radial plates with more than one perforation ; periproctal ring 

 formed by both radial and basal plates or only by the basal plates. 



Ambulacra narrow, straight, composed of a vertical row of 

 plates on either side of the median line ; plates low, broad, thick, 

 primaries only ; a vertical row of pairs of pores on each side of 

 an ambulacrum, a pair to each plate. 



Interradia with from four to five rows of plates diminishing at 



