144 THE CRINOIDEA 



We now proceed to review the sub-classes, orders, and families 

 of Crinoidea. 



SUB-CLASS 1. MONOCYCLICA, Bather (1899). 



Crinoidea in which the base consists of BB only, the aboral prolonga- 

 tions of the chambered organ being interradial ; new columnals are 

 introduced at the extreme proximal end of the stem. 



ORDER 1. Monocyclica Inadunata 

 ( = IXADUNATA, W. & Sp. pars, emend.) 



Monocyclica in which the dorsal cup is confined to the patina and 

 occasional intercalated anals ; such Amb or iAmb as enter the tegmen 

 remain supra-tegminal and not rigidly united. 



Thirty -one genera are here referred to this order. Of these, 20 

 diverge from the normal symmetry to a greater extent than by the intro- 

 duction of anals, viz. 10 through the horizontal bisection of certain RR, 

 other than r. post. R, usually r. and 1. ant. RR, while the remaining RR 

 often increase in width ; 8 through such increase in width of certain RR 

 (usually 1. post. R and ant. R), often accompanied by variation in the 

 number of arms directly springing from RR ; 1 by disappearance of a 

 radial (an occurrence also found in some of the other genera), and apparent 

 increase in number of arms springing from RR (as in some other genera). 

 Only 2 of the genera have regularly pinnulate arms, and in 15 the arms 

 are unb ranched. In 14, are not separated from one another or from 

 RR ; 1 1 at least have no anal tube, while in 6 of those the anus if it 

 existed must have pierced post. 0. In the rest the anal tube was always 

 supported by a well-defined median ridge of plates, simulating Br. In 

 Dicyclica Inadunata such characters as these are confined to very few 

 forms, and those the oldest. Therefore, as well as on anatomical grounds, 

 we infer that the prevalence of these characters denotes a primitive order. 



There is considerable parallelism of development between early In- 

 adunate forms of Dicyclica and Monocyclica ; nevertheless, most of the 

 Monocyclic genera fall into clearly marked groups, with which no Dicyclica 

 can be confused. The Hybocrinidae and Stephanocrinus are here taken 

 first, because of their simple structure and resemblance to Eublastoidea. 

 At a very early period the Heterocrinid type must have arisen, through 

 horizontal bisection of RR (antea, p. 112) and branching of arms. locrinus 

 and Anomalocrinus may be early offshoots from that line of descent ; 

 Herpetocrinus and the Calceocrinidae are other remarkably specialised off- 

 shoots, the latter surviving to Carboniferous times. Starting afresh from 

 the Hybocrinidae, the exaggeration of size in certain RR became more 

 pronounced in the Pisocrinidae, which, by addition of supplementary 

 radials and arms, lead up through Calycanthocrimis and Mycocrinus to 

 Catillocrinus, a type presenting a strange convergence to Halysiocrinus. 

 Zophocrinus, Allagecrinus, and certainly Haplocrinus appear to be aberrant 

 from this line of descent. The more regular Symbathocrinidae, with 

 their simple arms, represent either the direct descendants of the Hybo- 

 crinidae or a return from the Pisocrinidae to a more normal structure. 



