178 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



THALASSOMETRA PERIPOLOS A. H. Clark 



Plate 18, Fioures 56, 57 



Thalastometra peripolos A. H. Clark, Journ. Linn. Soc. (Zool.), vol. 36, No. 249, 1929, p. 635 (listed), 

 p. 651 (south of the Nicobar Islands, 560 fathoms; description; remarks), p. 653 (southeast of 

 Car Nicobar), pi. 40, figs. 3, 4. 



Diagnostic features. — There are no long curved spines near the distal edge of the 

 brachials; the cirri are XVII, 31-38, from one-quarter to nearly one-third the arm 

 length ; the arms (in the two known specimens) are 1 1 in number, the IIBr series being 

 4(3 + 4); the dorsal surface of the division series is thickly beset with excessively fine 

 spines which are often grouped in irregular bundles ; on the lower brachials these become 

 more scattered in the central portion of the dorsal surface; on the triangular and suc- 

 ceeding brachials the entire dorsal surface is beset with very fine spines, which become 

 longer on the slightly produced distal border. 



Description.- — The centrodorsal is low, flattened hemispherical, with the almost 

 flat dorsal pole finely pitted and finely spinous. The cirrus sockets are arranged in one 

 and a partial second crowded marginal rows, there being usually three at the margin 

 beneath each radial. 



The cirri are XVII, 31-38 (usually about 35), from 20 to 25 mm. long. The first 

 segment is very short, the second is slightly longer, the third is about two and one-half 

 times as broad as long, the fourth is half again as broad as long, and the fifth is some- 

 what longer than broad. The sixth, or more rarely the seventh, is a transition segment, 

 somewhat less than twice as long as broad, with the distal third white, in sharp contrast 

 to the dark brownish yellow proximal two-thirds. The following segments are similar, 

 but less constricted centrally. The segments succeeding slowly decrease in length 

 so that those in the terminal fourth of the cirri are half again as broad as long. On the 

 transition segment the distal edge dorsally is everted and slightly produced, and finely 

 dentate. As the segments become shorter this eversion of the distal edge dorsally 

 increases, becoming narrower and extending proximally so as to involve a triangular 

 area on the dorsal surface. In lateral view these processes appear as prominent blunt 

 dorsal spines arising from almost the whole of the dorsal surface of the segments with 

 the apex subterminal. On a few of the segments immediately preceding the penultimate 

 the process narrows into a small dorsal tubercle. The opposing spine is triangular, 

 arising from the entire dorsal surface of the penultimate segment, with the apex sub- 

 median to subterminal: its height is equal to about one-third the lateral width of the 

 segment. The terminal claw is about as long as the two preceding segments together, 

 and is evenly tapering and slightly and evenly curved. 



The radials are entirely concealed by the centrodorsal in the median line, but their 

 anterolateral angles are visible interradially as small rounded tubercles which usually 

 are armed with exceedingly short fine spines. The IBr! are very short, crescentic or 

 bandlike, becoming narrow laterally, and are in close lateral contact. Their surface 

 is thickly covered with excessively fine spines which are often grouped on irregular 

 pedicels suggesting paxillae. The IBr 2 (axillarics) are about three times as broad as 

 long, triangular or subrhombic, witli the lateral angles truncated and in close lateral 

 contact with those of their neighbors. The anterior edges are slightly thickened. 

 The entire surface is thickly beset with excessively fine spines which are more or less 

 grouped on low irregular bases. The division series are broad, in close lateral contact, 



