136 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



of Coccometra and Thysanometra, results from the shortness of the many component 

 segments, combined with the very strong bevel of their articular faces on either 

 side of the fulcral ridge, these two features causing the pinnulars to appear hex- 

 agonal in dorsal view or, if they are very short, rhombic. 



In many of the species of Comasteridje the pinnules on the arms arising from 

 the posterior rays, and often also on more or fewer of the arms arising from the 

 other rays, lack the ambulacral grooves and associated structures (fig. 1037, pi. 12). 



This is also true of the genital pinnules on all of the arms in certain species of 

 Charitometrida) (tigs. 1055, 1057, 1058, pi. 14). 



A curious condition occurs in C otnatuleUa hrachiolata (pi. 50, fig. 1332). All 

 of the arms possess ambulacral grooves, but in the proximal portion of the arms 

 single pinnules, or groups of two or sometimes of three, occur without grooves, and 

 in the outer part of the arms occasional grooveless pinnules are found which are 

 at once recognizable because of their greater stoutness. These grooveless pinnules 

 are capable of being coiled dorsally, and they are commonlj' found wrapped about 

 the organism to which the animal is clinging, quite after the manner of cirri. 



The pinnules of the macrophreate species are always cylindrical, and circular 

 in section or slightly flattened and more or less elliptical; but among the oligo- 

 phreate forms there is always a tendency to develop a longitudinal ridge along the 

 outer surface just distal to the median line. In the Calometridse (figs. 336, 339, 

 p. 229), Thalassometridae (figs. 337, 338, 343, 344, 350, p. 229), and Charitometridse 

 all the pinnules are affected and have assumed a .strongly prismatic form and tri- 

 angular cross section (fig. 659, p. 329, and part 1, fig. 54, p. 81) , with a sharp median 

 ridge which may be more or less serrate. In the Tropiometridse the oral and genital 

 pinnules are more or less jDrismatic, but not the distal, while in the other families 

 only the oral pinnules are modified, and these as a rule only slightly, showing a 

 tendency to develop long processes or spines on the distal ends of the segments at 

 the angles of the prism (fig. 315, p. 227), a ridge on the earlier pinnulars (figs. 267, 

 268, p. 207), or, in the Comasteridse, a terminal comb formed by the elongation of 

 one or both of the basal angles of the prism (figs. 594-657, pp. 309-327, and part 1, 

 figs. 56-58, p. 83, and 59, 60, p. 85). 



In the comatulids with 10 or more arms in which one or more division series 

 are present, and in those forms with undivided arms in which there are no IBr 

 series, the pinnules are arranged in pairs beginning with the first, so that the char- 

 acter of the pinnule on the second brachial is repeated in the first pinnule on the 

 other side of the arm, the character of the pinnule next above the first on the outer 

 side of the arm is repeated in the second pinnule on the inner side of the arm, and 

 so on. With the gradual loss of individuality in the genital pinnules this paired 

 arrangement becomes less evident, and there is no trace of it whatever in the distal 

 portion of the arm where exactly similar pinnules follow each other alternatingly 

 to the arm tip. 



In the genus Eitdiocrinus, in which there are but five arms, though 

 IBr series are present, the first pinnule takes the place of the additional arm 

 in the 10-armed forms (part 1, fig. 83, p. 136). The next pinnule, on the 

 other side of the arm. is the first pinnule of the 10-armed species; but this pin- 



