REPORT ON THE CRINOIDEA. 45 
complex in nature than that of Reteocrinus and Xenocrinus. But a transition between 
the two appears to be presented by some forms of Dendrocrinus, Heterocrinus, and 
Tocrinus. A little specimen figured by Meek, and referred to the aberrant type 
Dendrocrinus casei, shows the anal side very well.!_ Meek’s description runs as follows : 
— Anal series with the first piece resting directly upon the upper truncated side of the 
heptagonal posterior subradial (v.¢., basal) hexagonal in form, and bearing in direct 
succession above a series of hexagonal pieces gradually diminishing in size; while 
alternating with these similar small hexagonal pieces can be seen on each side of the 
mesial series, for some distance above the body between the free rays, and connecting 
with those of the ventral part.” His figure is a curious one, and does not quite agree 
with his description; for there seems to be a single large and pentagonal anal plate which 
separates two radials and rests in the angle formed by the upper edges of two basals 
(subradials, Meek). Upon this plate rests a series of seven gradually diminishing 
hexagonal pieces which stand out prominently from the smaller plates at their sides, 
just like the middle row of plates in the anal area of Reteocrinus nealli with which 
they seem to be comparable. If they supported a ventral sac like that of the typical 
Dendrocrinus, it was relatively much larger than that of Reteocrinus nealli, so that 
the vertical series of plates would end much farther from its summit than in that 
species. . 
Thus then in Onychocrinus, Taxocrinus, Reteocrinus, Xenocrinus, and even in 
Dendrocrinus casei the anal side shows this regular vertical series of plates which rests 
on a basal below and gradually diminishes in size. The only essential difference between 
it and the anal appendage of Thaumatocrinus is that it forms part of the body, being 
bound in with the rays by minute interradial plates which are not present in the simpler 
Thaumatocrinus. But this is often the fate of the lower pinnules in the Neocrinoids ; 
and it would assuredly also be the fate of an anal appendage in a Crinoid with the same 
calyx-characters as Thauwmatocrinus, but standing in the same relation to it as an 
extensively plated and multiradiate Comatula does to the naked and ten-armed Antedon 
rosaced. 
In the Cyathocrinoid genus Heterocrinus there appears to have been an anal append- 
age like that of Onychocrinus and Reteocrinus ; but it rested on the upper sloping sides 
of two adjacent radials instead of on a basal. 
In this type, as in the Cyathocrinidee generally, the capacity of the cup is compara- 
tively small, and the visceral cavity within the disk is almost entirely limited to its anal 
ce 
interradius, which is enormously enlarged, and forms the structure known as the “ ventral 
sac.” In Cyathocrinus itself this is a heavily plated tube, that commences at the upper 
edge of the “special anal” plate, above which its characteristic porous structure appears 
at once. But in Heterocrinus the ventral sac appears to be less robust, while the anal 
1 Paleontology of Ohio, vol. i. pl. iii. bis. fig. 2c, p. 29. 
