226 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Thalassometridse, where the interambulacral areas of the disk commonly contain 

 well-separated and often widely scattered plates. It is in the latter family, how- 

 ever, that the most perfect plating of the brachial perisome occurs. In Stylometra 

 (fig. 743, p. 349) the perisomic plates are more or less conical and have a spinous 

 appearance, a condition which is carried even further in Pcecilometra (fig. 1161, 

 pi. 26), though here the pointed ends are not so sharp. 



The Calometridse (figs. 734-737, p. 349) possess the most substantial plating 

 found in the recent crinoids, the plates being large and firmly united with each 

 other, so that the visceral mass is inclosed in a solid inflexible dome. This plating 

 is characteristic of the family and is developed at a very early age. It causes the 

 visceral mass to present a globular appearance which, on account of the rigidity 

 of the structure, is not affected by shrinkage in alcohol, so that a detached disk of 

 any of the species of this family is recognizable at sight. The calcareous plating 

 of the disk in the Calometridse is quite superficial, and it is firmly attached to the 

 subjacent soft structures only along the ambulacra, so that shrinkage on preserva- 

 tion merely causes the formation of cavities below the interambulacral areas of the 

 plated vault without in any way altering the external shape. In certain species 

 the dorsal angles of the lateral perisome just above the sutures between the radials 

 are protected by plates which are rather smaller than those on the ventral surface, 

 and each of these plates rises into a sharp conical protuberance (fig. 736, p. 349) 

 like the plates in Stylometra (fig. 743, p. 349, though rather more regular in form. 



In the disks of the species of the family Tropiometridse (figs. 729-733, p. 346) 

 no plating is ever visible. 



The disks of the species of Colobometridse (figs. 727, 728, p. 346) are usually 

 unplated, but in Colobometra diadema (fig. 728, p. 346) a complete pavement is 

 found, the interambulacral plates being especially large, while in 0. vepretum (fig. 

 727, p. 346) there may be plates about the base of the anal tube. 



In the genera Zygometra (figs. 710, 711, p. 346, and 1155-1157, pi. 25) and 

 Eudiocrinus (fig. 712, p. 346) the disk is completely plated, much as in the Calome- 

 tridse (figs. 734-737, p. 349), though the plates are less closely united and usually 

 somewhat smaller. In Catoptometra the plating is fairly complete, but is usually 

 more or less undeveloped in the interambulacral areas. 



Many, if not most, of the species belonging to the family Comasteridse (figs. 

 685-709, p. 341, 1142, 1144, 1147, 1148, pi. 23, 1149-1153, pi. 24, 1154, pi. 25, and 

 1165, pi. 26) develop large scattered irregularly rounded calcareous concretions in 

 the interambulacral areas, more particularly in the anal area ; but, contrary to what 

 is usually the case, these are commonly unaccompanied by prominent plates along 

 the sides of the disk ambulacra. Occasionally these concretions become so abundant 

 on and about the base of the anal tube as to form a fairly complete covering, though 

 as a rule they are sparsely distributed. 



In the Mariametrida3 (figs. 719-725, p. 346) there is no plating visible on 

 the disk except in the genus Mariametra (figs. 721-723, p. 346). In this genus the 

 disk is strongly plated and may be entirely covered by a pavement of small plates. 

 The disk in Pontiometra (figs. 724, 725, p. 346) appears naked, and for systematic 



