112 SMITHSONIAN INSTITUTION 



Order FLEXIBILIA Zittel 



Crinoids in which the lower brachials are loosely incorporated in the dorsal cup, either 

 by lateral union with each other, by interbrachials, or by a finely plated skin. Base dicyclic ; 

 infrabasals three, unequal, exceptionally fused, often greatly reduced or atrophied; some- 

 times fused with the top columnal. Radials and brachials united by a 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. 

 Arms non-pinnulate, uniserial, with a wide and shallow ventral groove. Tegmen flexible, 

 with exposed calcified ambulacra, roofed with movable covering plates; mouth supra- 

 tegminal and open. Orals small, asymmetrical, with food-grooves passing between them to 

 the mouth ; the posterior one much the largest and probably a madrepore ; they are more or 

 less surrounded by perisome, which often passes down between the rays. Stem circular; 

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

 expansion next to the calyx ; cirri usually wanting, or confined to the distal part of the stem 

 as branching rootlets. 



Geological range : Ordovician to Lower Coal Measures. 



The Flexibilia form a compact, well-defined group, limited to the Pale- 

 ozoic. They range from the earlier Ordovician (Trenton group) to the close 

 of the Lower Carboniferous, with a single genus passing into the Lower Coal 

 Measures. There is a general tendency from irregularity to regularity in the 

 calyx structure; the radianal, which is well developed in the lower positions in 

 the Ordovician and Silurian genera, becomes less conspicuous or disappears in 

 the Carboniferous. The calyx and arm plates are usually thick and relatively 

 short, with an imperfect muscular or loose ligamentous articulation which ad- 

 mits of much mobility. The combination of massiveness with flexibility is a 

 remarkable feature in the structure of the calyx wall. 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 corre- 

 sponding depression on the distal edge of those preceding; this extends but 

 little below the surface, the sutures beneath being perfectly straight. By con- 

 traction in fossilizing the thin projecting processes are frequently fractured, 

 giving rise to the erroneous idea 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 little differentiation between calyx and 

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

 the group less apparent than in the Camerata. The most prominent modifica- 



