62 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



Genital glands especially well developed in the posterior arms. Creeping forms 

 (see vol. 1, pt. 2, pp. 596 and 602 and following). The only family included in this 

 group is the Comasteridae. 



(2) Mariametrida. Centrodorsal more or less distinctly discoidal, very rarely 

 a low hemisphere. Centrodorsal cavity generally small and shallow. Radial 

 coelomic processes usually extending between the Centrodorsal and the radials, lying 

 in shallow depressions or meandering grooves. Cirri (as in the following groups) 

 always well developed. Cirrus sockets sometimes with an areola, otherwise without 

 sculpture, rarely in more than two alternating rows. Seldom any basal star. Basals 

 rarely protruding. Radial cavity large to moderate, usually filled by a central plug. 

 Muscular fossae on the articular faces of the radials of moderate size, sometimes 

 very reduced or even lacking. Synarthries usually well developed, only in the Zygo- 

 metridae partly replaced by syzygies. Arms generally more than 10. Number of 

 elements in the division series most frequently fixed (except in the Zygometridae and 

 Himerometridae) . No synarthrial backward projections. Synarthrial tubercules 

 in some of the Himerometridae. Syzygial septa numerous (except in small forms). 

 Syzygies occurring at rather long intervals on the arms. No side or covering plates. 

 Sacculi present in this and all the following groups. No terminal combs. Proximal 

 pinnules in the Zygometridae, certain Himerometridae, and the Mariametridae of 

 about the same length and unmodified; in the other families one or more pinnules 

 are distinguished by their length, thickness, or stiffness. Distal segments of the 

 pinnules in this and the following groups without hooks, or with rudimentary ones 

 only. Disk, as in the following groups, always endocyclic, rarely with calcareous 

 granules (Mariametra). Swimming forms. The following groups are included: 

 Zygometridae, Himerometridae, Mariametridae, Stephanometridae, Tropiometridae, 

 and Colobometridae. 



(3) Thalassometrida. Centrodorsal conical to discoidal, very variable. Cen- 

 trodorsal cavity rather deep. Cirri usually in columns. Cirrus sockets often some- 

 what sculptured (in contrast to all other comatulids except the Atelecrinidae). 

 Generally a basal star. Basals often protruding at the interradial angles. Muscular 

 fossae on the articular faces of the radials rather large, often narrow and high and bent 

 at an angle with the rest of the face. Radial cavity usually of medium size or small. 

 Calcareous plug inconspicuous or lacking. Arms usually wall-sided, and between 

 10 and 20 in number. Number of elements in the division series rather fixed (except 

 in Crinometra) . Synarthries well developed. Sometimes a syuartlirial backward 

 projection. Syzygial septa rather few. Interval between the syzygies variable. 

 Side and covering plates well developed. Arms usually laterally flattened distally, 

 in cross section triangular. The ends of the arms terminate abruptly with from 5 

 to 7 segments supporting rudimentary pinnules (in contrast to all other comatulids). 

 Proximal pinnules insignificantly specialized. Pinnule segments usually rectangular 

 or pronouncedly triangular (in contrast to all other comatulids). Disk often covered 

 with calcareous plates or granules. 



(a) Thalassometrida, in the restricted sense. No radial pits. Articular facets 

 of the synarthries narrow, occupying only a part of the synarthrial face. Thalas- 

 sometridae, Charitometridae, and Calometridae. 



