GENERA AJSTD GROUPS OF THE ECHINOIDEA. 151 



and ifc does not appear that he will do otherwise than hold his 

 ground. 



A. Agassiz retains the Lamarckian name Clypeaster for the 

 flat-based and more or less thin-edged Clypeasters, and it becomes 

 of great importance to discover whether the internal structures 

 of the tumid form are sufficient to necessitate its inclusion in a 

 different genus. 



This question was carefully considered before the publication 

 of this Eevision, and with the result that Clypeaster must remain 

 as a genus, and that two genera must be fouaded, one to include 

 the West-Indian Clypeaster reticulatusy Linu. sp. (^Ecliinanthus 

 rosaceiis, A. Agassiz), and the other to be associated with 

 Ecliijianfhus testudinarius, Grray, from Australia. 



^?^o^^^«Z«?^^A^^s, Bell, is a fourth genus closely allied to Clypeaster. 



It will be observed that EcMnanthus is now removed from the 

 Clypeastridse and is placed amongst the Cassidulidse, where 

 Breynius certainly meant it to be in the first instance. 



Genus Clypeaster, Lamk. 1816, pars. Hist. Nat. Anim. s. Vert. 

 vol. iii. p. 12. Desor, 18?8, Synopsis, p. 299. Liltken, 1863 

 (pub. 1864), Vid. Medd. f. Nat. For. i Kjohenh. pp. 100, 132. 

 A. Agassiz, 1872-74, Bevision, p. 306. Duncan, 1885, 

 Journ. Linn. Soc, Zool. vol. xix. p. 203, pi, 31. Loven, 1887, 

 EcJi. descr. hy Linnceus, pp. 171-176 (for synonymy). 



Test variable in size, moderate to large, subpentagonal, ovoid, 

 subcircular in marginal outline, usually truncated behind ; edge 

 thin, rarely slightly swollen, undulating in contour, with or with- 

 out re-entering angles. Dorsum very depressed or tall, conical 

 or subconical, or campanulate, and usually tumid centrally, and 

 sloping to the margins. Actinal surface flat, and with the central 

 peristome suddenly deeply sunken. 



Apical system small, central, or slightly excentric ; madreporite 

 central, hutton or star-shaped ; basal plates fused; genital pores 

 close to the edge of the madreporite, or in the median interradial 

 sutures, five in number ; radial plates small, and the pores also. 



Ambulacra much larger than the interradia, the petals large, 

 broad, long, tumid; poriferous zones broad, inclined, or narrow 

 and short, nearly closing distally ; plates low, broad, unsym- 

 metrical, compound; the adoral component a primary and the 

 aboral a low broad demi-plate ; pores unequal, wide apart, with 

 an intermediate groove ; pores in the transverse sutures of the 



