A MONOGRAPH OF THE EXISTING CRINOIDS 169 



which at its base (proximally) is not quite so wide as a single cirrus socket, from this 

 point gradually narrowing and disappearing between the distal sockets, which are 

 nearly or quite in apposition. 



The cirri are XX, 34-46, about 40 mm. long, and moderately stout. The longest 

 proximal segment is from two to two and one-half times as long as broad; the sixth 

 or seventh is a transition segment. 



The ends of the basal rays are visible in the interradial angles. Shallow and broad 

 subradial clefts are present. 



The IBi't are short, about four and one half times as broad as long, with the proxi- 

 mal and distal edges prominently everted and armed with about 6 or 8 irregular coarse 

 dentations which have numerous fine spines at their tips. The IBr 2 (axillaries) are 

 roughly rhombic with the lateral angles truncated and all the sides concave; the lateral 

 edges are nearly as long as those of the IBr, ; the proximal and distal edges are everted, 

 the proximal resembling the distal edges of the IBr^ the distal with a more regular 

 finely spinous margin. The elements of the IBr series are in close lateral apposition 

 and sharply, though narrowly, flattened against their neighbors; their lateral edges 

 are perfectly plain, without spines or tubercles. 



The 10 arms are all broken off at the first syzygy between brachials 3 + 4. In 

 shape the brachials resemble those of other species of the genus. They are perfectly 

 smooth, with no trace of spines or of median carination. The proximal and distal 

 edges are slightly thickened and everted, with a few small spines or tubercles. 



Notes. — The preceding description is based upon a single very fragmentary speci- 

 men from Investigator station 218. This species is related to Th. hawaiiensis and to 

 Th. hirsuta. From the former it differs in the much smaller centrodorsal, in the smaller 

 number of cirrus segments of which the proximal are much longer, in the shorter ossicles 

 of the IBr series which are in close lateral contact without intervening interradial water 

 pores, and in the eversion of the edges of the ossicles of the IBr series and of the earlier 

 brachials, these in Th. hawaiiensis being armed with large coarse blunt scattered 

 spines, though not as a whole turned outward. There are no spines on the dorsal 

 surface of the ossicles in Th. marginata such as occur, though sparingly, in Th. hawaii- 

 ensis. From Th. hirsuta this species differs in the very large and coarse instead of 

 fine tubercles on the dorsal pole of the centrodorsal, in the smoothness of the wedge- 

 shaped area separating the columns of cirrus sockets proximally, in the fewer cirrus 

 segments of which the longest are somewhat shorter, in the much less, and more coarsely, 

 spinous edges of the ossicles, and in the entire absence of a median carination and of 

 spines on the dorsal surface of the elements of the IBr series and lower brachials. 



The specimen from the Nicobar Islands is young with 10 arms 35 mm. long. 



Localities. — Investigator station 218; Maldive Islands; 384 meters [A. H. Clark, 

 1912, 1918] (1, I. M.). 



Nicobar Islands (lat. 7°52'38" N., long. 92°59'13" E.); cable repair ship Patrol, 

 Eastern and Associated Telegraph Co. [A. H. Clark, 1929] (1, B. M.). 



History. — Thalassometra marginata was first described in my memoir on the 

 crinoids of the Indian Ocean published in 1912 on the basis of a much mutilated indi- 

 vidual that had been dredged by the Royal Indian Marine Survey steamer Investigator 

 among the Maldive Islands in 210 fathoms. In my memoir on the unstalked crinoids 

 of the Siboga expedition published in 1918 this species was included in the key to the 



