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



resembling the Rhodocrinites in having five large plates separating the radials, 

 Thauviatocrinus differs from them and from most Palseocrinoids in the absence of any 

 higher series of calicular interradial plates resting upon the first series which separate 

 the radials. 



Except on the anal side the primary^interradial plates of Thaumatocrimts end simply in 

 a free rounded edge at the margin of the disk (PI. LVI. figs. 1-3, 5), which is doubtless due 

 to the simplicity of the arms ; for these become free almost at once, and are not connected 

 laterally by much perisome in which higher orders of radials could be supported. But in 

 the presence of the anal appendage on the azygous interradial (PI. LVI. figs. 2, 4, 5) 

 Thaumatocrinus bears a remarkable resemblance to Reteocrimis as understood by 

 Wachsmuth and Springer, and to the Xenocrinus of S. A. Miller ; while the appendage 

 has an even closer resemblance to the so called " anal series " of Omjchocrinus and 

 Taxocrinus, the lowest plate of which rests, not on a basal, but on the upper angles of the 

 two first radials. 



There can, I think, be no reasonable doubt that the anal appendage of Thaumato- 

 crinus, although free laterally, is homologous with the vertical series of plates in the anal 

 interradius of Reteocrinus and Xenocrinus, Onychocrinus and Taxocrinus. But owing 

 to the small size of Thaumatocrinus and the simplicity of its rays the anal appendage is 

 free ; whereas in the Palgeocrinoids it is united to the more or less branching rays by the 

 general series of minute irregular plates which occupy the anal interradius and pass 

 gradually upwards into those of the so called vault. 



It is difficult to consider the existence of interradials and of the anal appendage of 

 TJiaumatocrinus as instances of atavism, for no known Neocrinoid presents any similar 

 characters, and it is a long w\ay back from a recent Comatula to a Palseozoic Crinoid. 

 The reappearance of these characters in such a specialised t}'pe as a Comatula is conse- 

 quently not a little surprising. Associated with them we find the distinctly embryonic 

 characters of persistent basal and oral plates, the latter occurring in no other Comatula, 

 together with the simplicity of the undivided arms. 



Thaumatocrinus renovatus, P. H. Carpenter, 1883 (PI. LVI. figs. 1-5). 



Description of an Individual. — The total width of the calyx across the disk is barely 

 2 mm. ; and the height of the centro-dorsal and radials together is about the same. 

 The former (PI. LVI. figs. 1-4) is rounded below, with its central canal completely 

 closed up, so that it must have been detached for some little time from the remainder of 

 the stem. The bases of half a dozen cirri are attached to it, and there are pits for the 

 reception of two or three more. In the largest stump which is preserved (PL LVI. 

 fig^- 1)3) the first two joints are quite short, as is usually the case in aU cirri ; but the 

 third reaches a length of r5 mm., so that the cirri must have been very like those of 



