REPORT ON THE CRINOIDEA. 45 



complex in nature than that of Reteocrinus and Xenocnmis. 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 {i.e., 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 necdli 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 necdli, so that 

 the vertical series of plates would end much farther from its summit than in that 

 species. 



Thus then in Onychocrinus, Taxocrimis, 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 TJmumatocrinus 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 Thaumatocrinus, but standing in the same relation to it as an 

 extensively plated and multiradiate Comatula does to the naked and ten-armed Antedon 

 rosacea. 



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 ty^e, as in the Cyathocrinidse generally, the capacity of the cup is compara- 

 tively small, and the visceral cavity within the disk is almost entirely limited to its anal 

 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 



• Paleontology of Ohio, vol. i. pi. iii. his. fig. 2c, p. 29. 



