GENERA AND GHOrPS OF THE ECUTNOTDEA. / / 



The arrangement is represented in the recent forms of Eclii- 

 nothrix, to whicli genus the alliance is greater than to Astropyga. 

 But other distinctions of structure between, tlie genera are 

 evident. 



The fossil form lias not the ambulacra with the width and con- 

 vexity of those of EcTiinotJirix, and the median interradial area 

 near the apical system is tuberculate in Heterocidaris. 



G-enus Heterocidaris, Ootteau, 1860, Bull. Soc. Ge'ol. cle France, 

 ser. 2, vol. svii. p. 378 ; Ech. Foss. tie la SartJie, Suppl. p. 338, 

 pi. Ivi. Wright, 1860, Pal. Soc, Fcli. Ool. Form. p. 456. 

 {Amended.) 



Test large, circular in outline at the tumid ambitus, broader 

 than high, flat actinally, subconvex abactinally. 



Apical system absent in the specimens. 



Ambulacra very narrow, straight, not projecting, with two 

 vertical rows of very small perforate and crenulate tubercles. 

 Poriferous zones slightly depressed, narrow; pairs of pores in almost 

 straight series at and above the ambitus, although in triple com- 

 pound plates ; near the peristome the pairs of pores are in arcs and 

 oblique ; the tubercle of a compound plate is on the middle com- 

 ponent plate, and the sutures of the adoral and aboral component 

 plates and the middle plate are curved more or less, their con- 

 vexities directed towards the middle plate. Usually the com- 

 ponents are low, broad primaries ; but a demi-plate may occur 

 near the actinosome. 



Interradia very broad and tumid ; the plates low and broad, 

 carrying several rows of large, perforate, crenulate and scrobi- 

 culate primary tubercles separated by a rich granulation. The 

 vertical rows diminish above the ambitus ; but at least four 

 reach close to the apical system, especially the pair close to the 

 ambulacra, so that the median vacant space is very small and 

 insignificant. 



Peristome small, decagonal ; ambulacral ends small, with 

 branchial incisions on their flanks. Spines long, cylindrical, 

 striated longitudinally. 



Fossil. Inferior Oolite: England, Europe. 



M. Cotteau states that the auricular processes must have been 

 strong. 



