MONOGRAPH OF THE EXISTING CRINOIDS. 



89 



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

 are developed externally than interna IW. 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 



KiGS. 173-178. — Figures iLLnsTRATiNo arm structure of comatulids. 173. Distal arm division in 



SPECIMEN OP 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 BASE OF ARM IN HiMEBOMETRA MARTBNSI. 176. SINGLE ARM PAIR OF A 

 10-AKMED COMATULID. 177. BASH OF POST-RADIAL SERIES OP A 20-ARMED COMATULID SHOWING, HEAVILY 

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

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

 10-ARMBD 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 



