CRINOID FROM THE SOUTHERN SEA. 
923 
and third radials, though sometimes attaining a considerable relative size and 
importance, as in Guettardicrinus and some species of Apiocrinus. 
It is well known that many peculiarities which are merely transitory in young 
larvae of the Neocrinoids, are permanently retained in some of the Palaeocrinoids. 
This is the case, for example, with the primitive position of the anal plate within the 
ring of (first) radials of the larval Antedon. Thus in the Devonian genus Hexacrinus 
(Austin), and in some allied forms from the Carboniferous limestone among the 
Platycrindce, two of the five radials are separated permanently by a single large anal 
plate ; and the still earlier condition, before the radials have come into lateral contact 
at all, finds a parallel in the remarkable genus Reteocrinus* * * § from the Trenton and 
Hudson Paver groups (Lower Silurian) of North America. In this type, however, the 
radials are separated by what Messrs. Wachsmuth and Springer t describe as an 
“ interradial series resting directly upon the basals, consisting of a very large number 
of minute pieces of irregular form, and without definite arrangement.” A similar 
development of small irregular plates between the rays occurs in many Neocrinoids, 
both stalked and free, but the interradial series always commence at the level of the 
second or third radials, and are completely separated from the basals by the ring of 
united first radials. 
Now in Thaumatocrinus we not only find the primitive lateral separation of the 
radials to be permanent, as in Reteocrinus, but instead of the small and irregular 
interradials which rest on the basals of that type, Thaumatocrinus has one relatively 
large plate between every two radials (Plate 71, figs. 1-4, i, i). This is, as it were, a 
further development of the embryonic condition, but in a new direction. It is, how¬ 
ever, one which is not to be found in any Neocrinoid, either recent or fossil, and it is 
only among certain of the Palaeozoic Rliodocrinidce that a similar peculiarity presents 
itself. Messrs. Wachsmuth and Springer have grouped the genera in which it 
occurs into a special section, Rhodocrinites.\ They are Lyriocrinus (Hall) ; Rhipido- 
crinus (Beyrich); Thylacocrinus (Oehlert) ; Anthemocrmus ( W. and S.); Rhodocrinus 
(Miller); and Ollacrinus (Cumberland). All of them have a ring of ten plates resting 
on the basals, viz., the radials and five interradials of about the same size. This is 
well shown in the diagram of the calyx of Thylacocrinus (Plate 71, fig. 7), which 
1 have copied from that given by Oehlert. § 
While resembling the Rliodocrinites in having five large plates separating the 
radials, Thaumatocrinus differs from them, and from most Pakeocrinoids, in the absence 
of any higher series of calicular interradial plates resting upon the first series which 
* Of Billings, emend. Wachsmuth and Springer. 
f “ Revision of the Palaeocrinoidea,” Part II., p. 192. From the Proceedings of the Philadelphia 
Academy, July 26, 1881, p. 366. 
+ Ibid., pp. 182—184. 
§ “Description de deux nouveaux genres de Crino'ides du terrain devonien de la Mayenne.” Bull. 
Soc. Geo! de France. 3 e Ser., Tom. vii., p! 1., fig. 2. 
