MONOGRAPH OF THE EXISTING CRINOIDS. 



89 



of the genera of the Calometridae, one. or even two or three, more division series 

 are developed externally than internally. This arrangement is rather more stable 

 in the species in which it occurs than that just discussed, and is definitely char- 

 acteristic of many forms, while very frequent in others. The additional external 



FIGS. 173-178. FIGURES ILLUSTRATING ABM STRUCTURE OP COMATULIDS. 173. DISTAL ARM DIVISION IN 

 SPECIMEN OF TROPIOMETRA PICTA FROM Rio DE JANEIRO (DRAWING BY AUTHOR). 174. DIAGRAM ILLUS- 

 TRATING HOMOLOGIES OF POST-RADIAL OSSICLES IN GENUS CoMATELLA. 175. LONGITUDINAL SECTION 

 THROUGH DIVISION SERIES AND EASE OF ARM IN HlMEROMETRA MARTENSI. 176. SINGLE ARM PAIR OF A 

 10-ARMED COMATULID. 177. BASE OF POST-HADI 4L SEBIKS OF A 20-AKMED COMATULID SHOWING, HEAVILY 

 OUTLINED, OSSICLES HOMOLOGIES WITH THOSE OF A 10-AKMED SPECIES. 178. BASE OF POST-RADIAL SBKIES 

 OF A 30-ARMED COMATULID SHOWING, HEAVILY OUTLINED, OSSICLES HOMOLOGOUS WITH THOSE OF A 

 10-ARMED SPECIES. 



division series, like the additional internal just considered, are always composed 

 of two elements, but, contrary to what was found to be the rule in that case, they 

 are never developed except in species having all the division series of two elements. 

 The reason for the normal occcurrence of additional division series exter- 

 nally that is, along the outer branches arising from the IIBr axillaries is 



