136 PEOF. p. M. DTTlSrCAlSr's EEYISIOK OF THE 



Section I. 



Grenus Holecttpus, Desor, 1842, in Agassiz, Monogr. d'Ech. viv. 

 et foss., Des Galerit. livr. 3, pp. 52 & 63 (a group of Dis- 

 coidea). Desor, 1858, Synopsis, p. 168. Loven, 1874, Etudes, 

 pi. xiv. fig. 124. Cotteau, Peroii et Gauthier, 1880, Ech. 

 foss. de V Alger. 6 fasc. pi. vi. fig. 7. De Loriol, 1888, Faune 

 Cret. du Portug., EcTi. vol. ii. fasc. 2, p. 69 et seq. 



Moderate-sized Urcliins with a circular, or slightly pentagonal, 

 tumid ambitus ; subcorneal or depressed dorsally, tumid and 

 more or less hollowed for the central peristome. 



Apical system small, central, with five basal plates, the fifth 

 being sometimes small and without a duct-pore ; madreporite in 

 basal No. 2, extending centrally and separating the postero- 

 lateral basals. 



Ambulacra narrow, straight, widest at the ambitus ; pairs of 

 pores in simple series ; plates numerous, usually simple, small, 

 low primaries, some plates compound, and a demi-plate often 

 occurs aborally in some compound plates, which carry numerous 

 small primary tubercles. Tubercles, small primaries in many 

 vertical rows. 



Interradia with rather large plates, many vertical rows of 

 primaries, larger than those of the ambulacra. 



Peristome central, large, decagonal, with well-marked branchial 

 incisions. Pyramids of jaws slender, with a large foramen; 

 teeth slender, grooved (?). Perignathic girdle feeble and not 

 continuous. Periproct large, pyriform, in the posterior inter- 

 radium between the peristome and the posterior edge of the 

 test ; its plates largest posteriorly, smallest near the minute 

 anus anteriorly. 



Fossil. Oolites : England, Europe, N. Africa ; Cretaceous : 

 Europe, N. Africa, Asia, N. America. 



The increase of dimension of the madreporite and the presence 

 of the perforation in the fifth basal are characteristic of the Cre- 

 taceous species. 



Grenus Piletjs, Desor, 1858, Synopsis, p. 167. 



Large Urchins, pentagonal in marginal outline, tumid and sub- 

 hemispherical abactinally, more or less flat actinally. 



Apical system central, very small, with five basal plates, the fifth 

 imperforate, the madreporite in basal No. 2, and also separating 

 the postero-lateral basals. 



