306 COMPARATIVE ANATOMY CHAP. 



Order 2. Camerata. 



Plates of the calyx firmly connected by means of sutures. The apical capsule 

 shows a tendency to develop a very rich system of plates, incorporating the 

 proximal brachials to a greater or lesser extent. These brachials are connected 

 together in the interradii by interradial plates, which vary in number, and to which, 

 in the anal interradius, special anal plates may be added. In those cases in which 

 the arms are incorporated in the calyx to such an extent that they branch in the 

 latter before they become free from it, their branches may be connected by inter- 

 calated plates. Each of the five radials is usually followed by two brachial plates, 

 formerly called 2nd and 3rd radials. The tegmen calycis is richly plated with firmly 

 connected pieces, and is often much arched, forming a so-called vault. The mouth, 

 which lies in the centre of the tegmen, is covered with five firmly united oral 

 plates ; the hindermost of these, which is often the largest, projects in between 

 the four others. The ambulacra, with their lateral and covering plates, are mostly 

 not visible from outside, as the interambulacral plates which border them laterally, 

 and which are often very numerous, close together over them by means of processes, 

 and thus cover them externally. The ambulacra, in their course on to the bases of 

 the free arms, divide as many times as the arms have already divided on the 

 dorsal cup. The interradials of the dorsal cup often pass, without any sharp 

 boundary, into the interradially arranged interambulacrals of the tegmen calycis. 

 The subcentral (less frequently central) anus, which is surrounded by firm anal 

 plates, is either sessile or else comes to lie at the tip of a chimney -like prolongation 

 of the tegmen ; this anal tube, formerly thought to be a proboscis, may project 

 beyond the arms. Arms branched ; in adults, almost without exception, the 

 brachials become arranged in a double row with primitive articulation, and pinnules 

 closely folded together. Dorsal canals (in the brachials) have never been observed. 

 Exclusively paheozoic forms. 



Family 1. Reteocrinidse. 



Apical capsule, with monocyclic or dicyclic base. Four or five basals. Inter- 

 radial and interaxillary regions deeply sunk, plated with a large number of irregular 

 immovable pieces, which are continued on to the interambulacral areas of the tegmen 

 calycis. Posterior interradial region broader, and divided by a perpendicular row 

 of somewhat large anal plates. Anus subcentral. Arms composed of a single row of 

 calcareous joints. Pinnules strong. Ectcocrinus. Xenocrinus. 



Family 2. Rhodocrinidse. 



Apical capsule with dicyclic base. The circle of the five radials interrupted by 

 that of the five first interradials, which are in direct contact with the basals. 

 Interradial area plated with regular definitely arranged pieces. Posterior interradial 

 area differs but slightly. Tegmen calycis thickly plated. The plating of the apical 

 interradial region passes without break into that of the tegmen calycis. Ambulacra 

 not externally visible. Orals often indistinct. Anus subcentral. Ilhoducrinu*. 

 Gilbertsocrinus, Rhipidocrinus. 



Family 3. Glyptasteridse. 



Base dicyclic. With the exception of the first anal plate, which is in contact 

 with the posterior basal, the interradials do not touch the basals. Interradial 

 region of the apical capsule and tegmen calycis as in the Jikodocrinida: Oral plates 

 distinct. Anus subcentral. Glyptctster. 



