198 PEOF. p. M. DUlirCAH's REVISION OF THE 



Genus Htbocltpeus, Agassiz, 1839, EcTt. Suisse, pt. i. p. ^4. 

 WriffJit,lS56, Pal. Soc.,Ech.Ool. Form. p. 291, pls.xxi.«fe xxii. 



Test moderate in size, ovoid, usually highest in front, gibbous, 

 tumid, depressed, truncated and grooved posteriorly, swollen 

 actinally. 



Apical system subcentral, elongate ; tbe antero-lateral radial 

 plates uniting along the median line and separating tte pairs of 

 basal plates j a complementary plate or the iiftb basal plate may 

 separate the posterior basal plates ; the anal groove separates the 

 posterior radial plates. 



Ambulacra disjunct, similar, flush, apetalous, the anterior 

 sometimes in a groove and all more or less wavy. Poriferous 

 zones narrow, and the pairs are in simple series, and close abac- 

 tinally and at the margin, and wider apart and triserial at the 

 peristome ; plates low primaries abactinally, compound actinally. 



Interradia wide, tumid, and the posterior with a long deep 

 groove becoming wide and shallow at the truncated edge ; peri- 

 proct in the upper part. 



Peristome excentric in front, deep, pentagonal, and longer 

 than broad ; no floscelle. 



Tubercles close, uniform, small, perforate and crenulate ; scro- 

 bicules sunken. 



Fossil. England and Europe. 



It is said that a compact apical system may occur, a comple- 

 mentary plate being between the pair of basal plates, and a 

 similar plate is said to occur between the posterior pair of basal 

 plates. It may be a matter of doubt whether the generic cha- 

 racters can vary so much. 



Genus Iisteacltpeus, Gauthier, 1875, Coft., Teron Sf Gauthier, 

 Ech. Foss. de VAlgerie, 1876, fasc. ii. p. 23, pis. 14, 15. 



Test large, more or less hemispherical, somewhat depressed, 

 subcircular at the basal outline, slightly narrowest posteriorly, 

 flat actinally. 



Apical system subcentral, and at the highest point elongate ; 

 the antero-lateral radial plates uniting at the median line and 

 separating the pairs of basals ; posterior radials large, no fifth 

 basal ; the madreporite in the right anterior basal plate. 



Ambulacra subequal, flush, apetaloid, similar, increasing in 

 width to the margin, and thence diminishing towards the peri- 

 stome. Pairs of pores in single series ; pores small, oblique. 



