196.] Recent Literature. 2i»3 



As regards the major subdivisions of the stemmed echinoderms three 

 roups are recognized: the cvstids. the Mastoids, and the crinoids. 



5 groups of equal rank. The forms of the first 

 are earliest in time, lowest in taxoimnne [tuition, and are regarded as 

 the ancestral types of the other two. The crinoid type itself is a very 

 old one, dating from the Cambrian in which it is even then in a high 

 stage of development. During the Ordivician the cystidian features 

 had almost wholly disappeared. The crinoidal group is remarkable 

 for the persistence it has shown in preserving its pentamerous symme- 

 try ; and although the introduction of the anal plate so disturbed it as 

 to well nigh produce a permanent trilateral arrangement, the former 

 was finally permanently retained. 



Neocrinoidea and Pakeocrinoidea, the two primary groups of crinoids 

 which were formerly almost universally recognized, are abandoned. 

 In their stead are recognized three principal subdivisions : Inadunata, 

 Camerata and Articulata. It is quite remarkable that this ternate 

 grouping of the crinoids is essentially the same as Wachsmuth origi- 

 nally proposed more than twenty years ago, and that often being com- 

 pelled by students of the recent forms to abandon it and to substitute 

 others, a careful survey in the light of recent discoveries of all crinoids 

 both fossil and living has clearly shown that the main subdivisions first 

 suggested are essentially valid and are applicable to all known forms. 

 The criteria for separating the crinoids into orders are briefly as fol- 



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 



3. Growth of stem, whether new plates are formed beneath the prox- 

 imal ring of the calyx or beneath the top stem joint. 



The simplest forms, the Crinoidea Inadunata, have the dorsal cup 

 composed invariably of only two circlets of plates or three where infra- 

 basals 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 sub-groups, 

 the Larviformia at.d Fistulata. The former has the disk in ita simplest 

 possible form, being composed of five large orals arranged in a pyramid; 

 the second has the ventral side extended into a sac or closed tube often 

 reaching beyond the ends of the arms. 



