CLASS III CRINOIDEA 185 



niay Le present, even wlien the s[»ecial anal is wanting ; tlioy are interposed betvveen tlie 

 iiiteibracliials, following tlie median line of the posterior area. 



Urals ^The five large interradial plates which Surround the niouth or cover it. They are 

 Said to be symmetrical when of nearly tlie same size and form ; asymmetrical when the 

 posterior plate is pushed in betvveen, or is larger than, the other four. 



Ambulacrals = The rows of small plates in the tegmen which are radially situated. They 

 consist of adamhulacrals or side-pieces, and the covering-])latcs (Saumplättchen). The former, 

 when present, constitute the outer, the latter the inner rows of plates. The covering plates 

 form a roof over the food-grooves ; they are generally represented by two alternating rows 

 of small plates, more or less regulär in their arrangement, which are movable upon the arms 

 and pinnules, but upon the disk only in those Crinoids in which the mouth is exposed. 



The orientation is based upon the natural position of the Crinoid, with the arms upper- 

 most, and viewing the specimen from the anal side. The anal interradius will then be 

 posterior, the radius opposite to it anterior, while the right and left sides of it correspond 

 with right and left of the observer, 



Primary Divisions of the Crinoidea. 

 I. 



Crinoids in which tlie lower brachials take part niore or less in the dorsal cup. 

 All plates of the calyx united by close suture. Mouth and food-grooves closed. — 

 Order 1. Game rata. 



II. 



Crinoids in which the lower brachials are incorporated into the calyx either by 

 lateral union with each other, or by nieans of a skin studded with calcareous particles. 

 All plates from the radials up movable. Mouth and food-grooves exposed. Arms 

 non-pinnulate. The top stem Joint often fused with the infrabasals, and not always 

 the youngest Joint of the stem. — Order 2. Flexibilia of Zittel { = Ärticulata of W. 

 and Sp. non Miller and Müller). 



III. 



Crinoids in which the brachials are free above the radials. Plates of calyx united 

 by close suture. Mouth sub-tegminal. — Order 3. Inadunata. 



IV. 



Crinoids in which the mode of union of radials with the plates they l)ear is by 

 complete muscular articulation, and in which are combined the following additional 

 charäcters : open mouth and food grooves ; dorsal canals perforating radial and arm 

 plates ; uniserial arms only ; pinnules ; the general presence of a modified columnal, 

 or proximale ; the general absence of bilateral, and presence of pentamerous symmetry, 

 modified only by loss or addition of rays and not by anal structures. Brachials either 

 free, or more or less incorporated. — Order 4. Articülata. 



The first three of these divisions are represented in the Ordovician. The Gamerata 

 were the most specialised, and the first to disappear, being confined to the Paleozoic, 

 and becoming extinct in the Lower Goal Measures. The Flexibilia were similarly 

 limited. The Articülata ränge from the Mesozoic to the present time. The Inadunate 

 type, representing the most generalised structure of the Crinoids, is in its most 

 essential feature, though variously modified, carried forward with the Ärticulata^ and 

 thus has an unbroken ränge from the earliest Ordovician to the present. 



Order 1. CAMBRATA Wachsmuth and Springer. 



(Sphaeroidocrinacea Neumayr.) 



Crinoidea in which the lower brachials take part in the doisal cup. ylll plates of 

 tli£ calyx united hy close sutures, and immovahle. Mouth and food-grooves completely 



