296 CYSTOIDEA, BLASTOIDEA, AND HOLOTHUROIDEA. 



is a long and cylindrical column, and each plate carries a 

 series of respiratory pores or spongy tubercles, in lines 

 radiating from its centre. The genus is a link between the 

 Cystoidea and the Crinoidea. As before remarked, the last 

 unquestionable Cystideans belong to the Devonian period, 

 but there are strong grounds for placing here the genera 

 Oodaster and Codonites, which make a close approach in 

 many respects to Pentremites, and thus may be regarded as 

 transitional forms between the Cystoidea and Blastoidea. If 

 we adopt this view, as advocated by Billings, the range of 

 the Cystideans must be extended to the Carboniferous, as 

 both of these genera occur in deposits of this age. In the 

 curious Cadaster (subsequently changed to Codonaster with- 

 out sufficient cause) the calyx (fig. 175, A) is in the form 

 of a reversed cone, formed by three basals, surmounted by 

 a series of five larger plates. The upper surface shows five 

 recumbent arms, which radiate from a central aperture (oral 

 or ovarian (?) in function), each arm having a central am- 

 bulacral furrow, which is imperforate, and appears to have 

 really been bordered by a series of small pinnulse. These 

 furrowed arms are called " pseud-ambulacra " by M'Coy, but 

 as they have no connection with the respiratory tubes 

 (" hydrospires "), they do not correspond precisely with the 

 structures so named in the Blastoids. On the other hand, 

 there exist five large fissured areas, which correspond in 

 structure and function with the " pectinated rhombs " or 

 " hydrospires " of the true Cystoids, of which two are in- 

 complete, and are placed on each side of a large lateral (oro- 

 anal (?) or anal) aperture, while the other three are bisected 

 by the recumbent arms. 



In Codonites (fig. 175, B and c) the general form of the 

 calyx is much the same as in Cadaster, and the summit 

 carries five recumbent arms (or " pseud-ambulacra "), which 

 have a central furrow, and probably carried pimmlse, and 

 which agree with the corresponding structures in Cadaster, 

 and differ from those of Pentremites, in not being perforated 

 by any apertures. There is, further, no central orifice at the 

 point where the brachial furrows coalesce ; but there is a 

 large lateral aperture (supposed to be oro-anal) in one of the 



