REPORT ON THE CRINOIDEA. 161 



accidental one, and that I am inclined to attach too much importance to it ; and in that 

 case I should certainly agree mth Wachsmuth in calling them secondary interradials. 

 One point, however, is worth notice. The orals of Tliaumatocrinus, like those of 

 Hyocrinus (PI. VI. figs. 1-4 ; PI. LVI. fig. 5), are only separated from the calyx plates 

 by the thin band of perisome round the edge of the disk, which develops in the equatorial 

 zone of the larva, concurrently with the increase in the diameter of the cup. These 

 genera are thus in the condition of the advanced Pentacrinoid of Antedon rosacea, the 

 oral circlet of which, owing to the increase in the size of the visceral mass, becomes 

 detached from the summit of the primary radials ; and the space between the two series 

 is filled by the perisome of the equatorial zone, which separates the right and left larval 

 antimers. 



In the early stages of Hyocrinus, before this process had taken place, the orals would 

 rest directly against the upper edges of the radials, as they do in Holopus (PL III. fig. 2), 

 but in Tliaumatocrinus they must rest against the primary interradial plates. They 

 would thus be in precisely the same relative position as the " Scheitelstiicke " of Cocco- 

 crinus ; but I do not think that when a young Tliaumatocrinus is found in this condition 

 Wachsmuth will call the oral plates "secondary interradials"; though he uses this term 

 for the plates of Coccocrinus which I, like AUman and Zittel, consider as representing 

 the orals of Neocrinoids, including Tliaumatocrinus. 



If then the "Scheitelstiicke" of Coccocrinus be oral plates, there are strong 

 morphological reasons against the supposition that the ambulacra which pass in between 

 them were closed by small marginal pieces, i.e., by covering plates like those which occur 

 so frequently on the arms and pinnules of many Neocrinoids (PI. Vc. figs. 8-10 ; 

 PI. XIII. figs. 15, 16 ; PI. XVII. figs. 2-4, 7-9) and Palseocrinoids, and on the summit 

 of Cyathocrinus. Why should we invoke the existence of a "vault" over the orals of 

 Coccocrinus, simply because it is a Palseocrinoid l There is nothing of the kind in the 

 recent Holopus which dates back to the Chalk ; while the allied genera Cotylecrinus, 

 Eudesicrinus, and Eugeniacrinus are almost as old as the Mesozoic period. In no recent 

 Crinoid, nor in the Pentacrinoid larvae of those species which have covering plates to the 

 ambulacra do these plates rest upon the edges of the orals. They commence at the 

 margin of the peristome which is covered in by the orals, but there are none bordering 

 the edges of the grooves between these plates. 



It is partly, I believe, owing to his feeling the force of this objection that Mr. 

 Wachsmuth prefers to consider the " Scheitelstiicke " of Coccocrinus as secondary inter- 

 radials, rather than as orals. When he has found a specimen showing the small pieces at 

 their edges, I shall probably agree with him. But for the present I am inclined to lay 

 more stress on the resemblance of the " Scheitelstiicke " of Coccocrinus to the orals of 

 Neocrinoids ; although, according to Wachsmuth,' " this resemblance is probably merely 



1 Revision, part ii. p. 17. 

 (ZOOL. CHALL, EXP. — PART li^xn. — 1884.) Ii 21 



