REVIEWS 233 
The anal plates vary considerably in their position and distribution, 
and, in some groups are absent altogether. Asa rule they are largely 
represented in species with a stout tube or a lateral opening, and are 
wanting or are poorly developed when the anus is central. 
Among the Fistulata the term “anal plates” has been applied to 
two ossicles of different origin, the one radial, the other interradial. 
The latter is the homologue of the first anal of the Camerata, and rests 
upon the truncated posterior basal. The other which is not a supple- 
mentary plate but the lower section of the compound right posterior 
radial, performs anal functions only in certain genera. When both 
plates support the ventral sac asin most of the Poteriocrinide, the sec- 
ond, which is actually the first or lowest in point of position, is placed 
obliquely to the right of the other, without disturbing the orientation 
or the alternate arrangement with the basals. Both plates undergo 
many modifications, and the various phases as they occur in different 
geological stages, may be regarded as excellent criteria for generic sep- 
aration. ‘The earlier Camerata have neither a radi-anal nor a regular 
anal plate both of which make their appearance with the increasing size 
of the ventral sac. As this grows larger, the two posterior radials which 
previously were in contact laterally, part, and the anal piece is intro- 
duced to support the sac. Afterwards when the ventral sac attains 
still greater proportions, the supraradial is shifted to the right in a 
position almost directly above the right postero-lateral basal, so as to 
give to the infraradial which retains its place, a rather oblique direc- 
tion. Inthe Poteriocrinide, in which the lower faces of the costals fill 
up the whole width of the radials, leaving no room for attachment, the 
lower plates of the sac enter the calyx. At the close of the Carbonif- 
erous, the sac becomes reduced again to its former insignificance, the 
anal plates generally disappear, and the two posterior radials meet again 
laterally. This interpretation of the origin of the anal piece (or plate 
x as it is frequently called) differs essentially from that given by the 
English writers on the crinoids and particularly by Mr. Bather, who 
regards the plate as primitively derived from a brachial, which in time 
passed down from above into the dorsal cup. This author also claims 
that in the older forms with a compound right-posterior radial, such 
as in locrinus and Heterocrinus, the plate in question is supported by 
the supraradial and does not touch the infrabasal ; but that, further, 
in Hybocrinus and Dendrocrinus, it passes down from above the radial 
and finally rests with its lower half between the two posterior radials, 
