106 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Triangular processes. The iuterradial processes of the rosette (see figs. 577, 578, 

 pi. 10, and 589, 590, pi. 14, and pp. 320-322). 



Tripled dorsal spines. Dorsal spines which occur, three on each cirral, in a line at 

 right angles to the longitudinal axis of the cirrus (see fig. 348, p. 289). 



Trivium. In species of comasterids possessing ungrooved arms and primary bilat- 

 eral symmetry, the three anterior arms ; that is, the anterior, the right anterior, 

 and the left anterior arms (see pp. 1 1 0, 1 1 1 ); (see Bivium, Axis, and Orientation) . 



Trochitx. Fossil columnars, Considered individually. 



U. 



Underbasals. See Infrabasals. 



Ungrooved arms. See Grooveless arms. 



Unplated ambulacra. Ambulacra bordered by rudimentary side and covering 



plates not visible on ordinary examination, or by none at all. 

 Unplated disk. A disk upon which no epidermal calcareous plates are to be found 



on ordinary examination (see figs. 15-17, p. 67). 

 Upper surface. The surface of the animal, or the surface of any part of the animal, 



which is directed away from the ground or the* base when the animal is in 



its natural position. 



Thus the ventral surface of the animal as a whole is the upper surface. 

 Of the centrodorsal and the cirri, or the stem, or of their component parts, 



the proximal surface or surfaces are the upper, but of the other elements 



the distal. 



V. 



Ventral interradial furrows. The furrows on the ventral surface of the radial 



pentagon which lie over the interradial sutures (see figs. 453, p. 355, 464, p. 357, 



465-467, p. 359, 477, 478, p. 363, 488, 489, p. 365, 497, 499, 500, 501, p. 369, 



503, 505, 507, 508, p. 371, and 509-511, p. 373, and p. 374). 

 Ventral margin. Of the centrodorsal (see Inferior margin). 

 Ventral perisome. The perisome of the disk and of the ventral surface of the arms 



and pinnules. 

 Ventral spines. On the cirri; long overlapping spines sometimes developed on the 



distal midventral margin of the earlier segments. 



Ventral spines are very rare, but are well developed in the species of the 



genus Pterometra. 

 Ventral surface. See Adoral. Of the centrodorsal, that surface which is in contact 



with the radials (see figs. 229-234, p. 247). 

 Ventrolateral processes. The produced ventrolateral borders of the ossicles of the 



division series and of the first two brachials, as seen in Stephanometra and 



Cenometra (see fig. 87, p. 143). 

 Visceral mass. The central capsule resting upon the radials and the arm bases 



and bounded ventrally by the disk and laterally by the division series and 



so-called interradial areas. 



