GEITEEA AND GmOTJPS OE THE ECHINOIDEA. 213 



the ambulacra form the larger part of the margin ; membrane 

 with a central mouth surrounded by concentric buccal plates 

 without tentacles, largest at the margin. Periproct low down 

 in the posterior interradium, with a swelling below and more or 

 less of a hood above, elliptical, its membrane with concentric 

 plates. Below the periproct a mass of small miliary tubercles, 

 which may simulate a fasciole. Tuberculation scanty, of pri- 

 maries, with many secondary tubercles and miliaries ; and hence 

 a dense covering of very short spines, with broad bases. 



Becent. 422 to 1800 fathoms, Pacific, south of equator, to 

 Kerguelen, Magellan Strait, Caribbean Sea. 



G-enus CTSTECHiisrus, A. Agassiz, 1879, Proc. Amer. Acad. vol. xiv. 



p. 207; 1881, Beport 071 the' Challenger' Echini, p. 148. 



J. TV. Gregory, 1889, Quart. Journ. Geol. Soc. vol, xlv. 



Test very large, thin to rather stout, may be flexible, ovoid or 



elliptical in tumid marginal outline, tall, subconical or subhemi- 



spherical, and then low ; flat actinally, sunken slightly around 



the peristome, and tumid behind it. 



Apical system subcentral or slightly posterior, elongate, plates 

 confused; the pairs of basal plates separated either by large 

 radial or by accessory plates; the basals large, irregular, and 

 three, rarely four, perforated ; a small madreporite in the right 

 anterior basal ; radial plates confused, some small. 



Ambulacra similar, flush, apetalous, uniporous except close to 

 the peristome, widening to the ambitus, with, as a rule, large 

 plates, except close to the apex, where they are narrow. 



Interradia with large plates tending to become hexagonal, very 

 similar to ambulacral plates ; a small peristomial plate to each 

 area, and a meridosternum slightly tumid. 



Peristome circular or subelliptical, transverse, in front of the 

 centre, nearly flush, without lips, and with a raised internal ridge ; 

 peristomial membrane with concentric plates carrying small 

 spines ; mouth subcentral, in the membrane. Actinal ten- 

 tacles tufted. Periproct elliptical, varia,bly placed, marginal, and 

 just above or below it ; plates of membrane numerous. 



But few primary tubercles on the plates, whicli may have 

 ridges ; many secondary tubercles and miliaries ; spines short, 

 very close and equal, some serrate and cylindrical, others club- 

 shaped and spinnlose. Pedicellariae numerous, some on stalks, 

 tridactyle, bluut or sharp-headed, mostly trefoil-like and sharp. 



