■134 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In the Ptilometrinse and in the Calometridse, as well as in the short-armed 

 frenera of the Thalassometrinfe, the arms end very abruptly with sharply recurved 

 tips, and the terminal pinnules decrease in length very rapidly, so that the sub- 

 terminal pinnules extend for some distance beyond them (figs. 277. p. 213; 285, 

 p. 215; and part 1, fig. 46, p. 81). Thus when the arm is extended the tip shows 

 a broad V-shaped gap, with a blunted apex resting upon the subterminal brachials. 

 The first two segments of the middle and outer pinnules, and often also of 



the proximal pinnules, are always 

 shorter than those following (figs. 

 1059. 1063, pi. 14). The first is 

 usually between two and three 

 times as broad as long and more 

 or less crescentic in outline; the 

 second is trapezoidal, usually 

 aboiit as long as the proximal 

 (greater) width. 



It is at the well-developed 

 artictilation between these two os- 

 sicles that the flexion and exten- 

 sion of the pinnules takes place. 

 In some forms, more especially in 

 the Oligophreata, a considerable 

 amount of motion may be possible 

 between the more distal pin-, 

 nulars, but usually these articula- 

 tions are semirigid, and in cer- 

 tain types, as the Ptilometrinse, 

 C'alometridae, and many of the 

 Thalassometrinse and Charitome- 

 tridse, quite rigid. Motion be- 

 tween the pinnulars following the 

 second slowly increases in extent 

 toward the pinnule tip, and the 

 last four to six or more segments 

 are always movable even if the 

 rest of the pinnule be rigid. 



The first two pinnule seg- 

 ments are subject to but very slight variation in form, and their size in comparison 

 with the brachials which bear them is always approximately the same. Therefore 

 if the pinnules as a whole are very slender, as in most of the Macrophreata, they 

 appear disproportionately large (fig. 1063, pi. 14), not because they have actually 

 increased in size but merely because they have retained their normal and original 

 dimensions while the diameter of the segments following has decreased. 



Fig. 202. — Laterai, view op specimen of Pectinometra 



CARDUDM. 



