202 ECHINODERMATA— PELMATOZOA phylum iv 



Order 2. PLBXIBILIA Zittel. 



( = Articulata W. and Sp. non Müller). 



Crinoidea in which the lower brachials are incorporated in the dorsal cup, either 

 hy lateral union, hy interhrachials, or hy a finely plated sJcin, hut never rigidly. 

 Tegmen flexible, with amhulacra well defined, roofed with movaole covering plates ; 

 mouth supra-tegminal and open. Arms non-pinnulate, with a wide and shallow 

 ventral groove. Base dicydic ; infrabasals three, unequal, rarely iindivided, some- 

 times greatly reduced or atrophied ; often fused with top columnal. Radiais and 

 brachials united by modified muscular articulation, usually without transverse ridge, 

 accompanied by loose suture between other plates, producing a flexible calyx admitting 

 motion between apposed faces of the plates. Orals small, posterior one much the 

 largest, with the food grooves passing between them to the mouth ; they are more or 

 less surrounded by perisome, which often passes down between the rays. Stern round ; 

 proximal columnals usually very short, frequently wider than the others and forming a 

 Gonical expansion next to the calyx. Ordovician to Carboniferous. 



The calyx and arm plates in this group are usually thick and relatively short, 

 with a muscular or loose ligamentous articulation, which admits of much mobility 

 upon one another. The combination of massiveness with flexibility is a strong 

 distinctive character. The union between brachials is frequently marked exteriorly 

 by arcuate sutures, produced by a downward projection of the outer proximal edge of 

 the plates into a corresponding depression on the distal edge of those preceding ; this 

 extends but little below the surface, the sutures beneath being perfectly straight. By 

 contraction in fossilising the projecting processes are frequently fractured, giving rise 

 to an erroneous appearance of "patelloid plates." 



Owing to the fact that in most of the genera the rays are more or less continuous 

 from the radials up, with no well-defined zone of demarcation between calyx and 

 arms, there is a general similarity of type which renders the subdivisions less apparent 

 than in the Camerata. The most prominent modification of the general type relates 

 to the structure of the posterior interradius, upon which two well-defined divisions 

 may be recognised : (1) the strong anal side, in which the anal plates, when present, 

 are partly or whoUy incorporated in the calyx wall ; (2) the weak anal side, in which 

 the anal plates are separated from adjacent brachials at one or both sides by a pliant 

 integument, and tend to form a flexible series supporting the anal tube, which is 

 a mere extrusion of the pliant perisome of the tegmen. The second of these 

 structures is analogous to that in the larval stage of the living Crinoids, is the most 

 generalised, and was the most persistent — appearing in the Ordovician, and ending 

 with the culmination of this group in the Kaskaskia, or possibly Goal Measures. The 

 first is more specialised, and ranges from the Silurian to the Warsaw. Within it modifi- 

 cations in the size and position of the infrabasals, and in the general habitus, afford 

 ground for family divisions, which shade into one another to some extent. 



Among the genera of each family there may be observed a migration of the 

 radianal, from a primitive position directly underneath the right posterior radial (as 

 if the radial were transversely bisected) in the older formations (Ordovician and some 

 Silurian), to an oblique position to the lower left of the radial (some Silurian and 

 Devonian), and then to complete elimination in the Carboniferous. Modifications in 

 the number of primibrachs, by increase from two to three or more, were also to some 

 extent coincident with these changes. The structure of the base remains remarkably 

 constant, the proximal circlet consisting of three unequal infrabasals, which are 

 exceptionally fused to an undivided disk, and in certain genera tend to disappear by 

 resorption. 



