236 REVIEWS 
Not less important than the morphological contributions to a knowl- 
edge of the stemmed echinoderms are the advancements made in their 
classification, and it is safe to say that the systematic arrangement of 
the group is now practically settled for a century to come. 
The three groups of stalked echinoderms, the cystids, blastoids and 
crinoids are regarded as orders of equal rank. The forms of the first 
are earliest in time and lowest in taxonomic position, and may be con- 
sidered the ancestral types of the other two. The crinoid type itself is 
a very old one, dating from the Cambrian, in which it was already in 
a high stage of development. During the Ordovician the cystidian 
features almost wholly disappeared. The crinoidal group is remark- 
able for the persistency it has shown in preserving its pentamerous 
symmetry, and although the introduction of the anal plate was a 
disturbing element so great as to well-nigh produce a lasting bilateral 
_arrangement, the former type was finally permanently retained. 
The two primary groups of crinoids which were formerly almost 
universally accepted are abandoned. ‘These are the Neocrinoidea and 
Paleocrinoidea. In their stead are recognized three principal subdi- 
visions: Inadunata, Camerata and Articulata. It is particularly note- 
worthy that this ternate grouping of the crinoids is essentially the same 
as Wacksmuth originally proposed more than twenty years ago and that 
after being compelled by students of the recent forms to abandon it 
and to substitute others, a final careful survey, in the light of recent 
discoveries, of all crinoids both living and fossil, has clearly shown that 
the main subdivisions first suggested are essentially valid and are appli- 
cable to all known forms. ‘The criteria for separating the crinoids into 
orders are briefly : 
1. Condition of arms, whether free above the radials, or partly incor- 
porated in the calyx. 
2. Mode of union between plates of the calyx, whether movable or 
rigid. 
3. Growth of the stem, whether new plates are formed beneath the 
proximal ring of the calyx or beneath the top stem joint. 
The simplest forms of the Crinoidea Inadunata have the dorsal cup 
composed invariably of only two circlets of plates or three where 
infrabasals are present ; there are no supplementary ossicles except an 
anal piece which is however not always present; the arms are free from 
the radials up. In the construction of the ventral disk two different 
plans are recognizable and upon these are established two subgroups— 
