GENERA AND GEOUPS OF THE ECHINOIDEA, 171 



rather widest posteriorly, swollen dorsally, flat or pulvinate 

 actinallj, and may be subrostrated beliind. 



Apical system compact, witb four basal plates well perforated ; 

 tbe madreporite large and extending backwards in the usual basal, 

 separating the posterior lateral basals and touching the posterior 

 radial plates which are in contact. 



Ambulacra moderately broad, straight, flush, widest at the 

 ambitu?, narrowing dorsally and less so at the peristome ; plates 

 primaries, low, broad, each with a pair of pores ; pairs of pores 

 in simple straight series dorsally, continued to the peristome. 



Interradia with a moderate number of plates with small orna- 

 mentation. 



Peristome central and oblique, the long diameter being between 

 ambulacrum v. and interradium 2, sunken. Periproct infra- 

 marginal and marginal, triangular, with rounded angles. 



Fossil. Cretaceous : England, Europe, Africa. 



There may be some backward prolongation of the peristome in 

 some species of Caraiomus, and in M. Cotteau's valuable memoir 

 it appears that its long axis may be between amb. ii. and 

 amb. iv. 



The next genus is not a satisfactory one, for some of the 

 species are barely separable from Caratomus, and there is some 

 doubt about the generic position of the N.-Afriean forms with 

 semi-petaloid ambulacra, 



Grenus Pygaull's, Agassiz, 1847, Catal. rais., Ann. d. Sci. Nat. 

 vol. vii. p. 158. Desor, 1858, Synopsis, p. 251. Gott., Feron 

 Sf Gauth. 1876 and 1884, Ech. foss. de V Alger, new ed. of 

 fasc. 2, p. 75. 



Test small, thick, tumid at the ovoid or elliptical ambitus, low 

 and slightly convex dorsally, pulvinate actiually, subrostrated 

 posteriorly. 



Apical system slightly excentric in front. 



Ambulacra narrow, increasing in width to the ambitus and then 

 diminishing; poriferous zones narrow; pairs of pores in simple 

 series, and the shape of the pores of a pair may be different, 

 and either circular or elongate transversely. 



Peristome elongate, more or less oblique, and sunken, and the 

 pairs of pores are very slightly crowded at the margin. Periproct 

 inframarginal and ovoid-elongate or marginal. 



Fossil. Cretaceous : Europe, N. Africa. 



