THE CRINO1DEA 



175 



FIG. XC. 

 Analysis of cup of Giuocrinu*. 



or even the appearances of pores, between them (Fig. XXVI. 5). Arms 

 dichotomise 5-7 times, and in each series there are more brachials in the 

 admedian branch of the dichotom (Fig. LXXXIX. 1). The covering-plates 

 are well developed ; in their simplest form they are conical, in both out- 

 line and longitudinal section, regularly alternating, and each reaching 

 about ! across the ventral surface ; each covering-plate may, however, be 

 transversely divided, and the parts may come to be arranged in a manner 

 too complicated for description here (Fig. LXXXIX. 4). Stem round, 

 with lumen usually 5-lobed ; stem and lumen vary in width ; columnals 

 low, usually alternating in thickness and height, the smaller ones being 

 those last formed; joint-surface radiately striate (Fig. LXXXIX. 3); no 

 longitudinal sutures and no cirri. Gissocrinus, Angelin (1878 ; em. Bather, 

 1893), Silurian, Europe and possibly 

 America (Fig. XC. ; see also Figs. VIII., IX., 

 and XLL); connected with Palcwocrinus ; 

 one or two pairs of IBB usually fuse ; cup- 

 plates have clear axial ridges ; distal mar- 

 gins of Br usually project ; anal tube com- 

 pressed antero-posteriorly, its plates trans- 

 versely elongate and folded. Arachnocrinus, Meek & Worthen (1866), 

 Silurian to Devonian, America and Europe ; small cup and heavy arms, 

 which, together with anal tube, spread out horizontally from the cup. 

 Lecythocrimis, Miiller (1858, em. Zittel, 1879 = Taxocrinus briareus, 

 Schultze, 1866), Devonian, Eifel ; stem subquadrangular, with one large 

 central and 4 smaller peripheral canals (cf. Cupressocrinidae). FAMILY 6. 

 PETALOCRINIDAE. Cyathocrinoidea without x in dorsal cup, and with 



Fio. XCI. 



Petalocrinus. 1, partial recon- 

 struction of P. mirabilis, with one 

 arm removed to expose radial 

 facet, and other arms devoid of 

 covering-plates; the root is im- 

 aginary, x 2 diain. 2, section 

 across four grooves of an ami-fan, 

 showing traces of the original 

 sutures (s) between them, covering- 

 plates (c.p) closed and open, also 

 stages in the separation of the 

 axial canal (o.c) from the ventral 

 groove (v.g), compare Fig. VIII., 

 x 5 cliam. 3, articular facet of 

 arm -fan of P. visbycensis , ro, 

 muscle - fossa ; I, dorsal ligament- 

 fossa; r, fulcral ridge, x |. 4, 

 dorsal view of cup and proximal 

 regions of amis of P. mirabilis ; 

 Br, arm-fan ; St, proximal colum- 

 nal ; other letters as usual, x 3 

 diain. (3 and 4 are after Bather.) 



arms fused into solid arm-fans. Genus Petalocrinus, Weller (1896), 

 Silurian, Gotland and N. America, appears to have been derived from 

 Arachnocrinus by lateral fusion of the rami of each arm to form a blade 



