308 



COMPARATIVE ANATOMY 



CHAP. 



almost the whole of the apical capsule. Each radial is connected with a short and 

 small costal plate. The various brachials which follow (distichals, palmars, etc.) 



are free, i.e. belong to the freely out- 

 standing arms. In each interradius 

 there are at least three interradials, 

 which, however, appear more or less 

 shifted on to the oral side. In the 

 proximal (apical) interradial ring there 

 are no special anal plates, this ring 

 consisting in each interradius of 3-5 

 transversely placed plates, the central 

 one being the largest. Orals large. 

 Tegmen calycis mostly much arched. 

 The ambulacra and their covering 

 plates often appear at the surface. 

 Anus subcentral. Platycrinus (Fig. 

 254), Marsupiocrinus, Eudadocrinus. 



Family 7. Crotalocrinidse. 1 



Base dicyclic. The apical capsule 

 consists exclusively of the typical 

 plates of the apical system (infrabasals, 

 basals, and radials), to which is added 

 an anal plate. The brachials of the 

 separate rays (to the fourth order) 

 firmly united by sutures. Arms very 

 mobile, uniserial, long and much 

 branched ; branches free or connected 

 together in such a way as to form a net- 

 work around the calyx ; this network 

 is either continuous or else divided into 



five leaf-like lobes corresponding with the rays. Arms and their branches traversed 

 by large axial canals. Tegmen calycis flat, richly plated with distinct orals, iuter- 

 radials, and anals ; ambulacra externally visible, with large rigid covering plates, 

 which combine with the other plates to form the solid teginen. Anus subcentral. 



(This family is distinguished from all other Camerata by the presence of axial canals, 

 and by the mobility of the free joints of the arms.) Crotalocrinus, Eimllocrinus. 



CO 



FIG. 254. Platycrinus triacontadactylus (after 

 M'Coy). di, Distichals ; c, costals ; r, radial ; ba, 

 basal ; co, stem ; ir, interradials ; vk, ventral capsule. 



Family 8. Hexacrinidae. 



Base monocyclic. 2 or 3 basals. The first anal plate rests on the circle of 

 basals, and resembles the radials in shape. In other respects like the Platycrinida.'. 

 Hexacrinus, Talarocrinus, Dichocrinus. 



Family 9. Acrocrinidae. 



Base monocyclic. 2 basals, separated from the radials by a broad zone of 

 small plates arranged in circles round the basals ; these form the largest part of 

 the apical capsule. Each radial is followed by 2 costals. The radials and 



1 This family, originally placed near Cyathocrinus, was referred by Wachsmuth and 

 Springer, first to the Ichthyocrinoidse aud then to the Camerata ; Bather, however, 

 would refer it to its original position in the Inadunata. 



