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Genus— RYBOCLYVVS. 1839. 



The urchins grouped in this genus present an assemblage of characters which belong to 

 so many distinct forms of Echinoidea, that they may probably hereafter be found to 

 constitute a separate family, rather than a section of the EcHiNOcoNiDiE, with which they 

 are now provisionally placed for want of sufficient materials to justify such a separation. 



Their test is thin, and in general as wide as it is long; the anterior half is more 

 elevated than the posterior half, and sometimes rises into a prominent ridge. The surface 

 of the plates is covered with numerous concentric rows of small perforated tubercles, 

 set close together, and raised on low crenulated bosses, which are encircled by sunken 

 areolas, and the intermediate surface is crowded with microscopic miliary granules. 



The ambulacral areas are narrow, flexuous, and disjointed at the summit by the 

 length of the apical disc. The three anterior areas terminate at the front of the disc, 

 and the posterior pair at some distance behind them. The single area is lodged in a 

 depression of the anterior border. 



The poriferous zones are very contracted ; the pores are simple and unigeminal ; they 

 are placed close together on the upper surface, and wide apart at the base. 



The inter-ambulacral areas are very wide ; the single inter-ambulacrum is traversed 

 superiorly by a deep longitudinal valley, which commences behind the apical disc ; in the 

 upper part of this channel the wide vent opens on the dorsal region. 



The apical disc is central, but not vertical ; it is narrow and elongated, in consequence 

 of the length and singular disposition of its component elements ; the anterior pair of 

 ovarial plates are shield-shaped ; the right plate is the largest, and supports the madre- 

 poriform body ; the posterior pair are much larger and longer than the anterior pair, 

 and both pairs are perforated ; the single ovarial plate is composed of two or more long, 

 narrow, imperforate pieces, placed end to end in the centre of the disc ; the single ocular 

 plate is small ; the anterior pair are large, and disposed side by side in a line in the 

 middle of the disc, between the anterior and posterior ovarials, instead of being lodged in 

 angles formed b?/ them ; the posterior pair of oculars are situated at the end of the 

 posterior ovarials (PI. XXI, fig. 2 e), and thereby give lengthened extension to the disc. 



I have discovered this singular arrangement of the discal elements in three difierent 

 species, and now regard it as a valuable generic character (PI. XX, fig. 2 e ; PI. XXII, 

 fig. 1 e, and fig. 2 //). 



The small mouth opening, in general, is situated near the anterior third of the base, 

 which in some species is concave, and much undulated ; the peristome is sub-pentagonal 

 or elongated in the antero-posterior diameter. 



The Hyhodypi, like other Echinoconid^, have simple poriferous zones, perforated 



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