A MONOGRAPH OF THE EXISTING CRINOIDS 11 



A specimen from station 55 has the cirri XV, 20-21, from 3.5 to 5 mm. long; the 

 second segment has a small ventral spine, the third-fifth segments are twice as long as 

 broad and constricted centrally, and those following are shorter, from the tenth on- 

 ward about as long as broad or broader than long; from the sixth segment onward dorsal 

 spines are developed that reach about one-fourth the width of the segments in height. 

 The height of the opposing spine is equal to half the width of the penultimate segment. 



The radials are twice as broad as long with a median crest and small distal promi- 

 nences which are interradially fused and produced anteriorly separating the IBri. 

 The IBri are half again as broad as long with a median tubercle and latcrodistal promi- 

 nences; the IBr 2 , and first and second brachials bear similar, still larger, lobate flanges. 

 The IBr 2 (axillaries) are triangular, as long as broad, with a small cavity in the distal 

 angle. 



The 10 arms are 13 mm. long. The first two brachials are as high as broad, and the 

 first syzygial pair (composed of brachials 3 + 4) is also as long as broad. From the 

 fifteenth brachial onward a mediodorsal spine is developed. The left arm is often a 

 little longer and better developed than the right, though in one case the reverse is true. 



PI is 1 mm. long with 5 segments. P 2 and P 3 , like P b and P c , are usually lacking. 

 The distal pinnules are 2 mm. long with 10 segments. 



Orals are present, their length being one-fourth the radius of the disk; only the acute 

 angle remains, the base and the inner parts having been rcsorbed. 



Remarks. Professor Gislen wrote that in regard to the number of the cirrus seg- 

 ments this species is most closely related to Stenometra diadema, but it is well separated 

 from this species by the formation of the centrodorsal, the shortness of the cirri when 

 compared with the arms the latter are at least twice as long as the cirri and by the 

 large and many prominences on the ossicles of the arm bases. In this feature this 

 species approaches Daidalometra hana, the genus Daidalometra being properly distin- 

 guished from Stenometra only by a lesser number of arms. Professor Gislen considered 

 Stenometra dentata to a certain extent a transition form between Stenometra and Daidalo- 

 metra. 



Localities. Dr. Sixten Bock's Expedition to Japan, 1914, station 55; Bonin Is- 

 lands; eastnortheast of Chichijima; 210 meters; shells and sand; August 15, 1914 

 [GislSn, 1922, 1924, 1934]. 



Dr. Sixten Bock's Expedition to Japan, 1914, station 56; Bonin Islands; east of the 

 Channel; 210 meters; August 15, 1914 [Gislen, 1922, 1924, 1934] (1, U.S.N.M., E.llll). 



Dr. Sixten Bock's Expedition to Japan, 1914, station 59; east-northeast of 

 Anojima; 183 meters; August 15, 1914 [Gisldn, 1922, 1924, 1934]. 



Geographical range. Known only from the Bonin Islands. 



Bathymetrical range. From 183 to 210 meters. 



History. Stenometra dentata was described in detail in 1922 by Prof. Torsten 

 Gisl6n from nine specimens that had been collected by Dr. Sixten Bock in the Bonin 

 Islands in 1914, one at station 55, five at station 56, and three at station 59. In 1924 

 he discussed the arm division, synarthries, and syzygies of this species and figured the 

 synarthrial faces. In 1934 he listed it among the species with the Stenometra diadema 

 type of arm division. 



