THE CRINOIDEA 187 



ORDER 2. Flexibilia, Zittel 

 ( = ARTICULATA, W. & Sp. non Miiller). 



Dicyclica in which proximal Br are incorporated in the dorsal cup, 

 either by their own sides, or by iBr, or by a finely plated skin, but 

 never rigidly ; plates may occur between RR. Tegmen flexible, with 

 distinct Amb and numerous small iAmb ; mouth and food-grooves remain 

 supra-tegminal and open. The top columnal is a persistent proximale, 

 often fusing with IBB, which are frequently atrophied in the adult. 

 Arms non-pinnulate (Grade Impinnata), or pinmilate (Grade Pinnata), 

 but always uniserial. 



As in the case of the Distincta and Articulata among the Dicyclica 

 Inadunata, so the line between the grades Pinnata and Impinnata corre- 

 sponds roughly with that between Palaeozoic and Mesozoic time. But 

 in the present order the grades are more self-contained and the gap 

 between them greater. In fact, we are by no means certain that they 

 are rightly described as mere grades ; in other words, that the Pinnata 

 are the lineal descendants of Impinnata. The two divisions may have 

 arisen from Inadunata independently, springing from pinnulate and non- 

 pinnulate forms respectively. 



GRADE 1. Impinnata. 



Flexibilia, in which all plates of the crown are united by loose 

 suture or muscular articulation. IBB 3, the primitive r. post, remaining 

 as the small unfused IB. Br usually united by waving sutures, the lower 

 edge of each frequently with a projection that fits into a depression on 

 the plate below, and often becomes a separate patelloid plate. Arms 

 isotomous, or rami may bear ramules on one or both sides, but no 

 pinnules. Ventral groove wide and shallow ; axial canal separated from 

 it in proximal region. 5 O, between which food -grooves pass to the 

 mouth. Stem round ; proximal columnals very short, and usually wider 

 than the others. 



Many of the earlier genera can be distinguished from Dicyclica 

 Inadunata only by the greater thickness and more elaborate sutural 

 union of their plates, and the greater width and less length of the arms. 

 It is the combination of massiveness with flexibility that characterises 

 the Grade. There is never an elaborate anal sac. Within the Grade 

 Impinnata can be traced the evolution of heterotomous arm-branching of 

 two types, also an increase in number of iBr. The genera seem to 

 merge into one another, and are as yet too ill-defined to be grouped into 

 families on a sure genetic basis. The following arrangement represents 

 similarities of structure rather than lines of descent. 



FAMILY 1. ICHTHYOCRINIDAE. Impinnata with no iBr, with isotomous 

 arms, closely abutting by their sides. Genera Pycnosaccus, Angelin (1878; 

 syn. Oncocrinus, Bather), Silurian, Gotland, England, and N. America, has a 

 cup like that of Barycrinus, with x and RA, and with strong axial folds 

 (Fig. CVIL); arms, though abutting above, are distinct below and do 



