CRINOIDEA. 



277 



The origin of the radianal is supposed to be as follows. The right 

 posterior radial (p. 272) is sometimes divided horizontally into an upper 

 (oral) and lower (aboral) part. These are called the superradial and in- 

 ferradial respectively. The inferradial is often shifted so as to lie in the 

 anal interradius, between the right posterior radial (now superradial) and 

 the special anal plate ; it is then called radianal. This horizontal 

 division of the radial may appear in other radii, but no shifting of the 

 lower element occurs except in the radius specified. 



The skeleton of the arms (Fig. 193). The arms are supported 

 by a row of plates called brachials. They are placed on the 

 abactinal side and are often grooved on their actinal or ventral 



FIG. 193. Caetocrinus proboscidatis Hall. Calcareous Limestone, Iowa. A, the calyx 

 cover is broken away, so as to show the subtegrainal skeleton ; a the covering pieces of 

 the ambulacra! grooves. The opening of the ambulacra! grooves are shown on the 



interradial ; the plates marked (ft on each side of the anal interradial are interradials. 

 C, cast of a calyx-cover showing the impressions of the ambulacral canals a leading to the 

 mouth o ; an anus. D covering plates of an ambulacral groove (from Zittel, after Meek 

 and Wortheu). 



. tt 



surfaces for the reception of the soft parts of the arms ; more- 

 over they contain a canal for the axial cord of the apical nervous 

 system (Fig. 197, 8). The first brachial of an arm is carried by the 

 corresponding radial plate of the apical system. When the arms 

 divide once dichotomously, the brachials before the division are 

 called primibrachs (costals) (Fig. 193, r 2 , r 3 ) and the brachial im- 

 mediately before the division is called the axillary primibrachial 

 or primaxil (r 3 ). When the arms divide a second time the 

 brachials which occur between the first and second divisions are 

 called secundibrachs (distichals), the axillary secundibrach being 



