236 PART II. SYSTEMATIC ACCOUNT 



the keels of the bridle and by the absence of double rings in the tube. 



Material: c. 15 tubes, some containing animals. 



Localities : the southwestern part of the Bering Sea, to the east and south- 

 east of Cape Olyutorski (60°N 170°20'E) and also to the east of the Koman- 

 dorski Islands (55°N 166°E). 



Depth: 1693-4811 m. 



16. Siboglinum hyperboreum Ivanov (Fig. 120) 



Ivanov, 1960c: 5, 27, 30, 98, 105, 128, 134, 160, 170-2, 174, 177, 185, 188, 197, 263, 

 Figs. 87, 120. 



In August, 1957, at a depth of 215 m in the Greenland Sea, R.V. OF took 

 a rather large number of tubes of Siboglinum, shown by immediate study as 

 all belonging to the same undescribed species. The fore-part of the body is 

 four and a half to five times as long as it is broad and possesses two annular 

 grooves, one immediately in front of the bridle, and the other a little behind 

 the point of attachment of the tentacle (Fig. 120Д 5, C). This latter is long 

 and rather slender, and quite devoid of pinnules. The conical cephalic lobe is 

 not large. The keels of the bridle are of medium thickness, fused together 

 ventrally and with a wide gap between their dorsal ends (Fig. 1205). Under 

 the microscope it may be observed that along the light-brown cuticular strip 

 of the bridle stretches a very delicate dark linear crest (Fig. 120D), similar to 

 that found in S. tenue. The mesosoma broadens out towards its hind end, 

 where its hind edge is curved backwards on the dorsal and notched in the 

 mid-ventral line. 



The transversely oval papillae of the metameric part of the trunk are small, 

 and the ventral sulcus between them is deep and narrow. The dorsal ciliated 

 band, of the usual width, begins at the very front of the metasoma (Fig. 

 1205). Of the three girdles, the first has a broad gap on the ventral side, the 

 second is broken dorsally, while the third, lying a considerable distance 

 behind the other two, is interrupted both dorsally and ventrally (Fig. 

 120£, F). All three lie on well-developed, muscular ridges, and a few 

 rounded papillae with crescentic plaques on their tips are situated near the 

 last girdle. The yellowish spatulate biserial toothed platelets have the anterior 

 group of teeth only a little less strongly developed than the back group, and 

 between the two groups lies a considerable interval bare of teeth (Fig. 120G). 

 The platelets for the most part are between 13 and 14ju long and up to 4/a 

 broad, but occasionally one may find an aberrant platelet only 10/x long. 



The tentacle is more than 5 mm long, the fore-part of the body 0-6-0-9 mm 



