310 BUliiETIN 82, UNITED STATES NATIONAL, MUSEUM. 



(2) studded with a few large rounded isolated concretions, mostly in the anal area 

 (figs. 741, 743, 747, 750, 753, p. 349), (3) thickly beset with very numerous subequal 

 rounded plates of moderate size which are usually more or less in contact (figs. 

 738, 744, 746, p. 349), or (4) almost comj^letely, or quite, incased in a pavement 

 of larger and thinner polygonal plates (figs. 734-737, p. 349). 



The disk is quite naked in most of the Comasteridse, in the Himerometridse 

 and Stephanometridse, in most of the Mariametridae, most of the Colobometridse, 

 most of the Antedonidae, and most of the Pentametrocrinidce, and in the Tropio- 

 metridas and Atelecrinidae. 



The disk is studded with a few large rounded isolated concretions, for the 

 most part in the anal area, in many of the ComasteridEe (particularly in the 

 Comactiniinse) , and in a few of the larger species of Antedoninoe (chiefly in the 

 genera Anted on and Mastigometra). 



The disk is thicldy beset with very numerous subequal rounded plates of 

 moderate size, which are usually more or less in contact, in the Thalassometridae and 

 Charitometridse, and in Pentametrocnnus semperi and P. aflanticus of the Penta- 

 metrocrinidse. 



The disk is entirely inclosed in a solid vault of large polygonal plates, which 

 separates very easily from the underlying structures, in Eudiocrinus of the Zygo- 

 metridae, G olobometra diadema of the Colobometridse, and in the Calometridse ; the 

 same type of plating, though less perfect and with the plates not always in contact, 

 occurs in Zygometra and Catoptometra of the Zygometridte, and in Mariametra 

 of the Mariametridse. 



In the following families the disk is always of the same type : Himerometridae, 

 Stephanometridae, Tropiometridae, Atelecrinidse, Thalassometridas, Charitometridse, 

 and Calometridae ; while the following include species having the disk of two types : 

 Comasteridse, Zygometridse, Mariametridae, Colobometridae, Antedonidae, and Pen- 

 tametrocrinidse. 



The occurrence of different types of disks in related forms is emphasized by 

 species in the following genera: Coloiometra, Antedon, Mastigometra, and Penta- 

 metrocrinus ; and different types of disks are even found in different individuals of 

 species in the genera Coviatula, Antedon, and Mastigometra. 



The plating of the disks of the species of Calometridse is quite characteristic, 

 and detached disks of these species (fig. 736, p. 349) can at once be assigned to the 

 family; but this is the only family in which this is the case. The disks of the 

 species of Thalassometridae and Charitometridae present features not otherwise 

 known in the comatulids, but those of the different species in the two families show 

 only average differences. Disks of the species of Eudiocrinus are readily recog- 

 nizable, more because of the very narrow torn areas than on account of the character 

 of the plating. 



In the remaining comatulid families and genera overlapping and intergradation 

 of the disk characters occur to such an extent that no general statements can be 

 made, though in many types certain features other than the plating, or combinations 

 of characters, are diagnostic. For instance, the disks of the species of Promacho- 



