A MONOGRAPH OF THE EXISTING CRINOIDS 203 



Chondrometra robusta A. H. CLARK, Journ. Washington Acad. Sci., vol. 6, No. 17, 1916, p. 608 (listed); 

 Unstalked crinoids of the Siboffa-Exped., 1918, p. 188 (in key; range), p. 189 (references; notes; 

 station 297), fig. 9, p. 189, p. 275 (listed). 



Diagnostic features. The centrodorsal is sharply conical, as high as broad at the 

 base, with the cirrus sockets arranged in 5 more or less regular often partially double 

 midradial columns; the arms are 170-211 mm. long; and the cirri are long with numer- 

 ous segments, up to 60 mm. in length with up to 28 segments. This may be only the 

 fully mature form of Ch. aculeata. 



Description. The centrodorsal is long, conical, about 6 mm. broad at the base 

 and 6 mm, high. The cirrus sockets are confined to the midradial regions where they 

 are arranged in two very closely crowded converging alternating columns, which merge 

 and become a single column distally. There are four or five cirrus sockets in each radial 

 area. 



The cirri are stout and very long, XX-XXV, the peripheral with 26-28 segments 

 from 55 to 60 mm. in length, those near the apex of the centrodorsal with 21 segments 

 and from 35 to 40 mm. long. The first segment is short and those following increase 

 in length becoming approximately as long as broad on the sixth and from the eighth 

 onward about twice as long as broad. The distal dorsal edges of the segments are some- 

 what thickened, especially in the outer half of the cirri. The penultimate segment is 

 slightly less in diameter than those preceding. The opposing spine is very small, 

 terminal. The terminal claw is about as long as the penultimate segment, slender, 

 and slightly curved. The cirri are moderately compressed laterally. 



The 10 arms are 211 mm. long. The arm bases and arms are essentially as in 

 C. aculeata, but much more rugged. The IBr 2 (axillary) has a prominent dorsoventrally 

 elongate well-rounded tubercle. The second brachial is similar. The following brachi- 

 als as far as the fourteenth have prominent rounded median tubercles as long as the 

 segments. After the fourteenth brachial these tubercles become high, thick, overlap- 

 ping spines, the bases of which occupy the entire dorsal median line of the segments. 

 Terminally these spines gradually decrease in height and eventually disappear. 



The genital pinnules are expanded as in Glyptometra tuberosa. 



Notes. Two smaller specimens were secured with the type just described. One 

 of these is about the size of the type of C. aculeata, from which it is easily distinguished 

 by the much longer and more sharply conical centrodorsal and by the dorsal ornamenta- 

 tion of the arms which, though not so developed as in the fully grown, is yet prominently 

 marked. The cirrus segments are longer than those of C. aculeata. The other speci- 

 men has the arms only 55 mm. long. The dorsal ornamentation is only just beginning 

 to appear, but is already of the type characteristic of the fully grown. The peripheral 

 cirri are XIV, 19-20, 21 mm. long. The centrodorsal is of the characteristic shape. 



The specimen from Albatross station 5349 is large and resembles the type. 



Of the specimens from Siboga station 297 one, with the arms about 170 mm. long, 

 very closely resembles the type; it is very slightly smaller and more slender, and the 

 median ornamentation of the arms is slightly less pronounced, though of exactly the 

 same character. 



Another specimen is somewhat smaller than the preceding with the dorsal orna- 

 mentation somewhat less marked; the cirrus sockets are arranged in a single regular 



