GENEEA AND GROUPS OF THE ECHINOIDEA. 197 



Ambulacra disjunct and similar, tlie anterior in a slight groove 

 or flush, the others flush, increasing in width to the ambitus, the 

 pairs of pores in low primary plates; posterior ambulacra at 

 varying distances from the anal groove. Interporiferous areas 

 with small perforate and crenulate tubercles. 



Interradia very unequal, broad ; the postero-lateral areas 

 uniting at the dorsum behind the postero-lateral basal plates. 



Actinally the postero-lateral interradia are symmetrical, there 

 being no fusion of plates. The interradial ornamentation is of 

 the same kind as that of the ambulacra, but closer. Peristome 

 pentagonal or subcircular, its margins formed by two unequal 

 plates to each ambulacrum, and a single interradial plate to each 

 area. Neither phyllodes nor bourrelets. Periproct posterior, 

 in a groove at varying heights above the margin. 



Fossil. Oolite : England and Europe, IN". Africa. Cretaceous : 

 Europe. 



Grenus Dtsasteh, Agassiz, 1839, Ecli.foss. d. I. Suisse, vol, i. p. 1 ; 

 1847, Ann. d. Sci. Nat. vol. vii. p. 275 (Disaster). Desor, 

 1842, Monogr. d'Ech. viv. etfoss. livr. 3, p. 7. Cotteau, 1885, 

 Ech. foss. de F Tonne, vol. i. p. 335 ; 1867-74, Pal Frang., 

 Terr. Cret. vol. ix. p. 106. 



Test moderate, subcylindrical, long, low, rounded in front, 

 truncated squarely posteriorly. 



Apical system dislocated ; the anterior part excentric in front, 

 with the two pairs of basal plates in contact, the anterior and 

 antero-lateral radial plates being external ; the posterior part 

 consisting of the postero-lateral radials united along the median 

 line and separated from the anterior part by numerous small 

 plates of the postero-lateral interradia ; the madreporite in the 

 right anterior basal plate, or extending between the postero- 

 lateral basal plates. 



Ambulacra disjunct, flush, increasing in width to the ambitus, 

 the posterior may be flexuous ; bivium and trivium widely sepa- 

 rated ; pairs in simple series. Postero-lateral interradia large, 

 may have additional plates at the dorsum. 



Peristome excentric in front, subpentagonal, plain ; a single 

 peristomial plate in each interradium. Periproct pyrif orm, in the 

 posterior truncation, supramarginal. 



Fossil. Europe, N. Africa. 



This genus is distinguished from Collyrites by having the basal 

 plates not separated by antero-lateral radial plates. 



