THE CRINOIDEA 



135 



which the term " centro-dorsal " was originally applied, and to which 

 it must be restricted (e.g. Antedon, Eudiocrinus, Thaumatocrinus ; see 

 Figs. CXVII.-CXIX.). These forms anchor themselves by their 

 cirri, and though capable of crawling, climbing, and swimming, do 

 not often exercise their faculty of locomotion. Secondly, the group 

 in which either a portion of remaining stem, or the lower part of 

 the cup (i.e. BB or IBB), becomes solidified, usually by additional 

 deposition of stereom, into a knob, which, one may suppose, serves 

 as ballast or as a sea-anchor ; such forms are Agassizocrinus (p. 181), 

 Edriocrmns (Fig. CXIL), and Millericrinus Pratti (Fig. LIL). Both 

 of these groups have a small calycal cavity with thick walls, and 

 there can be little doubt but that all are attached by a stem in the 

 earlier stages of ontogeny. The third group, comprising Marsupites 

 (Fig. CIV.), Saccocoma (Fig. LXVIIL), and Uintacrinus (Fig. CIIL), 



Fia. LIL 



Stages in the loss of the stem by 

 Millericrinus Pratti. 1, cnp with stem 01 

 seventy coluinnals ( x 3). 2, distal end of a 

 stem, with apparent root (natural size). 

 3, cup with fairly long stem, with inter- 

 calated new coluninals (natural size). 4, 

 c.up with stem of twenty coluinnals (x :]). 

 5, cup with stem of live coluinnals ( x 2). 

 <i, lower part of crown, with stem reduced 

 to a pentagonal plate (c), with slight 

 trace of atrophied next columnal (x i). 

 7, base, closed below by a single plate 

 (c), with no trace of lumen or of other 

 coluinnals. This plate is the proximate 

 (p of 3 and 4), but is covered by second- 

 ary stereom (x _>). (All after P. H. Car- 

 penter.) 



has no trace of a stem or of any anchoring structure, but is in all 

 respects adapted for free locomotion ; the calycal cavity is large in 

 proportion to the thickness of the arms, and is enclosed by thin 

 flexible walls. Of^ these three genera, Saccocoma is the most special- 

 ised, and is supposed by Jaekel (1893) to have been pelagic, living 

 in swarms. Uintacrinus, with its extraordinarily long and movable 

 arms, may also have been pelagic. The genera of this third group, 

 although of origin as diverse as those in the other groups, resemble 

 one another in the presence of a central, pentagonal, apical plate. 

 This in Saccocoma may be the fused basals ; in Uintacrinus and Marsu- 

 pites it represents neither basals nor infrabasals, but may be the 

 proximale, or the supposed distal columnar plate ("dorso-central"), 

 or a new supplementary plate. It is safest to call it centrale. 



Another curious modification, perhaps connected with a free- 

 floating existence, was presented by the root of Scyphocrinu*. 

 This swelled out into a hollow, chambered, balloon-like body, 

 referred by Barrande to an independent class of Echinoderms under 



