42 THE VOYAGE OF H.M.S. CHALLENGER. 
of Apiocrinus, are continued upwards on to the surface of the disk between the 
ambulacra. Wachsmuth and Springer described this ventral disk of Onychocrinus and of 
other Ichthyocrinide as “ composed of a more or less soft or scaly integument, yielding 
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 shall 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 Thawmatocrinus would 
assume, did it exist in a larger Crinoid such as Pentacrinus asteria (Pl. XIII. fig: 1), 
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 (Pl. VI. fig. 42> PL eV fe 96 5 6PL SX VE. 
hess JET. fie 25 Pl XXXTX. fie, 25 Pls LY.), 
Tt 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 Ichthyocrinide 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 Yaaxocrinus and Onychocrinus seem to 
have had an anal appendage like that of .Zhaumatocrinus, 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 Heterocrinus, Hall; Reteocrinus, 
Billings ; and Xenocrinus, Miller, all from the Lower Silurian of America.’ 
According to Wachsmuth and Springer’s definition of FReteocrinus, the posterior 
1 Revision, part i. p. 31. * Revision, part i. p. 31. 
* 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, partii. p. 191; Amer. Journ. Sci. and Arts, April 1883, p. 256). Ishould say, however, 
that 8S. A, Miller differs from his fellow-workers in America upon this subject, and refers the species grouped under 
Reteocrinus by Wachsmuth and Springer to, at least, three genera (Amer. Journ. Sci. and Arts, 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 Tamocrinus, 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). 
- 
