312 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



D. Greatly elongated cirri tapering to a sharp point; the distal segments are 

 elongated and without dorsal processes; there is no opposing spine and the teiminal 

 claw is nearly straight. 



Comasteridse (the emphasis at B) A, B 



Zygometridae ; Antedonidse A, B, C, D 



Himerometridse (the emphasis at B) B, C, D 



Stephanometridae ; Mariametridse ; Colobometridse (the empha- 

 sis at B) B, C 



Tropiometridse A 



Calometridse B 



Thalassometridae (the emphasis at C) B, C 



Charitometridse (the emphasis at A) A, B 



Pentametrocrinidae; Atelecrinidse A, D 



The interrelationships between the various types of cirri and of centrodorsals, 

 and the relations of both to the larger systematic groups, are briefly shown in the 

 following table: 



A. The primitive type of centrodorsal. 



B. Thick discoidal or columnar centrodorsals, tending to become more or less 

 conical; the cirrus sockets are in columns, three or more to each radial area, but the 

 radial areas are not marked off from each other. 



C. Columnar or conical centrodorsals, with the surface distinctly marked off 

 into radial areas ; the cirrus sockets are in three columns in each radial area. 



D. Columnar or conical centrodorsals, much reduced in size; the surface is 

 sharply differentiated into radial areas; the cirrus sockets are in two columns in 

 each radial area. 



A. Short, stout and smooth cirri, with a small number of similar and subequal 

 segments. 



B. Longer cirri, with more numerous segments, of which the distal are shorter 

 than the proximal and bear dorsal processes. 



C. Enormously elongated cirri, with the same structure as those grouped 

 under B. 



D. Greatly elongated cirri tapering to a sharp point; the distal segments are 

 elongated and without dorsal processes ; there is no opposing spine and the terminal 



claw is nearly straight. 



Centrodorsal. Cirri. 



Comasteridae A A, B 



Zygometridse A A, B, C, D 



Himerometridse A B, C, D 



Stephanometridae; Mariametridae; Colobo- 

 metridse A B, C 



Tropiometridse A A 



Calometridse A B 



Thalassometridffi B-D (D) B, C 



Charitometridse . . A-C (B-C) A, B 



Antedonidas.. A-D (A) A, B, C, D 



Pentametrocrinidse A A, D 



Atelecrinidae C-D A, D 



