42 THE VOYAGE OF H.M.S. CHALLENGER. 



of Apiocrinus, are continued upwards on to the surface of the disk between the 

 ambuhxcra. Wachsmuth and Springer described this ventral disk of Onychocrinus and of 

 other Ichthyocrinidas as " composed of a more or less soft or scaly integument, }delding 

 to motion in the body and arms ; " ^ and they regard it as homologous with the more 

 solid vault of Platycrinus and Actinocrinus. I believe, however, as I shaU explain 

 more fully further on, that this was the real ventral surface of the Crinoid and not 

 a " vault " at all ; while the so-called proboscis or anal tube with the small interradials 

 round it is just in the condition which the anal appendage of Thaumatocrinus would 

 assume, did it exist in a larger Crinoid such as Pentacrinus asteria (PL XIII. fig. l), 

 with a well-plated perisome between the rays. This plating may be continued up on to 

 the disk and to the summit of the anal tube (PI. VI. fig. 4 ; PL XVII. fig. 6 ; PL XXVI. 

 figs. 1,2; PL XXXIV. fig. 2 ; PL XXXIX. fig. 2 ; PL LV.). 



It unites the lower arm divisions closely together ; and any additional appendage 

 in the anal interradius would naturally be bound in with it, just as the four to seven joints 

 of the anal appendage in Onychocrinus are bound in with the numerous minute pieces 

 between the rays. But I see no reason for supposing that such an appendage would 

 form part of the tube up to its opening, and be in any way grooved on its inner 

 side. For it seems to taper away rapidly and to become merged into the general 

 plating of the anal interradius in the flexible vault, or disk as I should call it. The 

 passages quoted above both from Meek and Worthen, and from Wachsmuth and 

 Springer, would admit of this interpretation ; and in the first case at any rate, it seems 

 (from the context) to be the one which was intended. But Wachsmuth and Springer 

 also speak of the anus of the Ichthyocrinidas as " unknown except in Taxocrinus and 

 Onychocrinus, which have a small lateral tube." '^ This observation refers to the small 

 appendage already mentioned ; but it must not be understood to imply (as it well 

 might) that this appendage is hollow and pierced by the rectum. 



Some older Crinoids, however, than Taxocrinus and Onychocrinus seem to 

 have had an anal appendage like that of Thatimatocrinus, which was sometimes 

 surrounded by numerous minute interradial pieces, so as to form a support to the anal 

 side of the disk between the rays. I mean the genera Hetcrocrinus, Hall ; ReLcocrinus, 

 Billings ; and Xenocrinus, Miller, all from the Lower Silurian of America.^ 



According to Wachsmuth and Springer's definition of Reteocrinus, the posterior 



1 Revision, part i. p. 31. ^ Revision, part i. p. 31. 



3 As regards the second of these, I shall speak of it in the sense in which it is used by Wachsmuth and Springer, 

 Wetherby, and others (Revision, part ii. p. 191 ; Amer. Journ. Sci. and Arts, April 1883, p. 256). I should say, however, 

 that S. A. Miller differs from his fellow-workers in America upon this subject, and refers the species gTouped under 

 Keteocriwus hy Wachsmuth and Springer to, at least, three genera {ATner. Journ. Sci. and ^rts, August 1883, p. 105 ; and 

 Journ. Cine. Soc. Nat. Hist, vol. vi., December 1883, pp. 217-230). As, however, all the species referred by them to this 

 genus appear to possess an anal appendage like that of Thaumatocrinus, Onychocrinus, and Taxocrinus, it is obviously 

 more convenient to consider them all as congeneric, as I am also inclined to do for other reasons (see Phil. Trans., 

 1883, pp. 923-933). 



