MONOGRAPH OF THE EXISTING CRINOIDS. 331 



separated from the radials by suliu-e, are not separated from the centrodorsal (figs. 

 234, p. 247, and 250, p. 253). Externally these tubercles have all the appearance of 

 true basal rays, but on dissection they arc found to bo merely interradial eleva- 

 tions on the ventral sui-face of the centrotlorsal, exteriorly marked by a tubercle, 

 but forming an integral part of the centrodorsal and in no way separable from it. 

 These pseudobasal rays may be joined about the central cavity of the centro- 

 dorsal by a pentagonal raised area, just like the radial accessory bridge, which, as 

 explained above, often joins the inner ends of the basal rays; but, like the pseudo- 

 basal rays themselves, this structure is only an integral part of the centrodorsal, 

 not adhering to the radials, as do the true basal rays and tiicir dependent structures. 

 I have found these pseudo-basal rays to be especially prominent in Coccometra 

 Jiagenii (fig. 284, p. 261) and in Eudiocrinus ornatus (fig. 250, p. 253), and, without 

 the radial connective, in certain of the PentametrocrinidsB (fig. 299, p. 264). 

 Structurally these are part of the centrodorsal and are in no way distinguishable 

 from it, but morphologically they are true basal rays, developed for the purpose of 

 fillLng up the gap caused by the dorsal interradial furrow, though tlieir substance 

 merges unperceptibly into that of the centrt)dorsal instead of being more or less 

 sharply differentiated from it. There is no distinct line of demarkation between 

 the pseudo-basal rays and true basal rays, all intergrades bemg found within the 

 family Antedonidse, and apparently even within certain species of that family. 



Ra<!ianal. 



Hitherto the radianal plate, though a fundamental structure in many fossil 

 forms, has been supposed to be unrepresented m the recent types. The penta- 

 crinoid young of the comatulids possess a plate in the anal interradius, situated 

 between the two posterior radials, to which the name of anal has been universally 

 applied, on the assumption that it is the homologne of the so-called anal x of the 

 fossil forms (figs. 553, pi. 5, 560, 563, pi. 6, 576, pi. 9, 588, pi. 13, and 594, 596, 

 pi. 16). 



Now all the work previously done upon the developuig crbioid has been based 

 upon one or other of the species of the genus Antedon, one of the most specialized 

 of the genera in the group to which it belongs, and hence one of the least satis- 

 factory for purposes of phylogenetic uivestigation. 



Exammation of a fine series of the young of Protnachocrinus ierguelensis has 

 brought out many points which the larvse of Antedon do not show, and, among 

 other things, has made it clear that the so-called nual of the young of the coma- 

 tulids is homologous not with the anal ,r of the fossil tj-jjcs, but with the radianal. 



In Antedon the so-called anal plate is formed, at about the period of develop- 

 ment of the IBr,, between the two posterior radials: but is it noticeable that while 

 the radial to the left of it is of normal shape that to the right has its left siile more 

 or less cut away for its reception (fig. 563, pi. 6). \Ylien the "anal" is lifted out 

 from the circlet of radials just ])re^-ious to its resorption it is noticeable that it 

 .keeps to tho right of the posterior hiterradial area, remaining more or less hi con- 

 tact with the right-hand radial and first primibrach uistead of being drawn directly 

 upward, as would be expected (fig. 553, pi. 5) ; also the right radial is a.symetrical, 



