216 PEor. p. M. Duncan's eevision of the 



radium. The tubercles are well developed, wide apart, scro- 

 biculate, and are over the whole surface of the test, and there 

 are small close granules. 



Fossil. Eocene (Oligocene) : Europe. 



Genus Stenonia, Besor, 1858, Synopsis, p. 333. 



Large conical Urchins, with the periproct inframarginal, a bila- 

 biate peristome, and with equal ambulacra. The apical system is 

 compact. The ambulacral plates are about half of the height of 

 the interradial plates. 



Fossil. Upper Chalk: Europe. 



There is but one species known of this remarkable genus, which 

 Desor pointed out is Fchinocorys {Ananchytes) with a compact 

 apical system ; he notices that the test is very thick, and that 

 there is a bulging of the centres of the plates. 



II, Family SpATANaiD^. 



Tests ovoid or cordiform, longer than broad, with numerous 

 plates, and usually with an anterior groove. 



Apical system with four or less perforated basal plates, com- 

 pact, or with the madreporite variable in its posterior extension ; 

 radial plates five and external. 



Ambulacra in a bivium and trivium, the anterior differing in 

 shape and construction from the others, which may be petaloid 

 dorsally or apetaloid, biporous, or uniporous ; the postero-lateral 

 long actinally and bounding the actinal plastron. Peripodia 

 large around the peristome, forming rudimentary phyllodes, the 

 tentacles being penicillate there, and either simple or branchial 

 above. 



Pairs of pores of the petaloid parts differing from the others. 



Interradia narrow at the apex and at the peristome, where each 

 has a single plate: the posterior plate the largest, and form- 

 ing a labrum more or less projecting behind the transversely 

 elliptical peristome. The postero-lateral interradia are usually 

 unsymmetrical actinally, and the actinal plastron of the posterior 

 area may be amphisternous or meridosternous. 



Periproct in the posterior interradium. Eascioles present or 



absent. Spines slender. Spheridia most numerous in the bivium. 



Unisexual or bisexual, undergoing free metamorphoses or not. 



