&ENEEA AND ftROITPS OP THE ECHINOIDEA. 209 



Grenus HEMipisrEtrsTES, Agassiz^ 1840, Gat. Syst. p. 2 ; 1^4i7,Ann. 

 d. Sci. Nat. Tol. viii. p. 31. Desor, 1858, Synopsis, p. 348. 

 Loven, 188S, Fourtalesia, pp. 70, 92, 95. 



Test large, ovoid iu marginal outline, liigli and tumid, and raised 

 or not in front abactinally, flat actinally ; narrowly and deeply 

 grooved anteriorly and more or less emargiuate posteriorly. 



Apical system central, elongate ; the antero-lateral radial plates 

 large and uniting at the median line ; the madreporite diffused. 



Ambulacra dissimilar ; the anterior in the groove, its plates 

 and pairs of pores numerous and small, the rows of pores wide 

 apart ; the paired ambulacra fl.ush, semipetaloid, more or less 

 curved, open distally, with numerous plates ; the pores of the 

 pairs circular and elliptical and elongate. 



Interradia actinally symmetrical on account of the fusion of 

 the second plates of both of the postero-lateral areas. Posterior 

 interradium with a wide produced labrum and a meridosternous 

 plastron with large plates and a zigzag of sutures. 



Peristome excentric in front, much sunken, crescent-shaped, 

 broad. Perij)roct supramarginal. A small equal granulation, 

 large on the edge of the anterior groove. 



Fossil. Cretaceous : Europe, Africa. 



Grenus Caediaster, Forbes, 1852, Mem. Geol. Survey, Decade iv. 

 pi. ix. Desor, 1858, Synopsis, p. 344. 



Syn. Stegaster, Pomel (pars) ; Cihaster, Pom el (pars). 



Test large, cordiform, slightly convex actinally, tumid at the 

 margin and rather depressed abactinally, broader than high, may 

 be high anteriorly ; with a well-marked anterior groove keeled at 

 its sides, reflected more or less on the dorsum. 



Apical system subcentral or excentric in front, elongate, the 

 anterior basals being separated from the postero-lateral by the 

 united antero-lateral radials. Madreporite in the antero-lateral 

 basal plate. 



Ambulacra apetalous and biporous, flush with the surface of 

 the test except the anterior, which is in the anterior groove. 

 Anterior ambulacrum with small pores, the pairs becoming wider 

 apart vertically and horizontally, from increased dimensions of 

 the plates, and reaching far down the groove, before they become 

 smaller. The posterior poriferous zones of the other ambulacra 

 have larger pairs of pores than their anterior zones ; pores elon- 

 gate, and often placed obliquely in the pairs. 



LINN, JOUEN. — ZOOLOGY, VOL, XXIII. 14 



