MORPHOLOGICAL PART. 63 
monocyclic, and the uppermost stem joint as the last formed plate of the 
stem. But he seems to have changed his opinion afterwards, for in a letter 
to us, dated November 28, 1890, he writes as follows: “I have spent this 
morning studying my material of Rizocrinus, including some fresh specimens 
which I found among my father’s collection, and I conclude that you are 
right in considering the top-stem-joint as a centro-dorsal like that of Bowr- 
gueticrinus and the Apiocrinide. It is, however, always the smallest, 7. e., 
thinnest joint of the stem. But it is distinctly marked into five fosse for the 
basals by radial ridges, and this is sometimes very apparent in the younger 
individuals. See Chall. Rep. Plate X. Figs. 7 and 8. It is more marked in 
RL. Rawson than in &. lofotensis. The pentagonal space in the centre, Plate 
X. Fig. 5, is nothing but the axial canal.” He then continues; “ Bathycrinus 
is a very different form, with its very numerous thin joints at the top of the 
stem (see Plates VII. and VIII), and I have been thinking for some time 
past that it must be removed from the Bourgueticrinide and made the type 
of a new family. The ten arms, and the trifascial articulation are also good 
characters, as the latter replaces the syzygies of Rhizocrinus.” 
Except in one point, we fully agree with this. Examining Plate X of 
the Challenger Report, we find in Rhizocrinus the space within the basal ring 
very different from the axial canal in the stem joints (see our Plate VI. Figs. 
7a, ), the former being fully twice as large, and pentangular, while the canal 
is oval. The space between the basals is radially disposed, as in the Apiocri- 
nide, and we believe that Rhizocrinus is also pseudomonocyclic. Its structure 
is altogether different from that of Bathycrinus and Hyoerinus, which in our 
opinion are true monocyclic forms. They have no centro-dorsal, and the 
uppermost joint is the last formed plate of the stem. The open space within 
the basal ring is interradially disposed, and has the form and size of the 
stem canal. 
Our observations on the Apiocrinide led us in 1885* to the conclusion 
that most of the Neocrinoidea may have possessed rudimentary infrabasals in 
their larval state. This rather startling statement was unfavorably received 
by Carpenter, and severely criticised by him.| He held at that time that 
with the exception of Enerinus, Extracrinus, Marsupites and Uintacrinus, all Meso- 
zoic and recent Crinoids were monocyclic. His objections, however, did not 
seem to us convincing, and in Section I. Part II. of the Revision, pp. 294- 
* Revision, Part III., Section I., pp. 8, 71. 
t Bibliogr. Notices. Ann. and Mag., November, 1886, p. 408. 
