SIBOGLINUM 111 



10 (13) The dorsal ends of the keels of the bridle are somewhat thickened. 



The groove between the protosoma and the mesosoma is simple. 



11 (12) There is a glandular girdle behind the bridle, interrupted dorsally 



and ventrally. The pinnules of the bipectinate tentacle are very 

 thin. The toothed platelets of the girdles are 17-23/n long. The tube 

 is segmented, with 12-18 rings per segment, and a large part of it 

 carries double, more or less anastomosing rings. 



4.S.fedotovi(p. 193). 



12 (11) There is no glandular structure behind the bridle. The pinnules of 



the bipectinate tentacle are thickish. The toothed platelets of the 

 girdles are 1 0— 1 1 /x long. The tube is segmented, with 8 rings per 

 segment; the rings are nowhere double. 3. S. pinnulatum (p. 190). 



13 (10) The dorsal ends of the keels of the bridle are not thickened. The 



groove between the protosoma and the mesosoma is curved. 



14 (15) The groove between the protosoma and the mesosoma lies at a 



considerable distance from the bridle and is strongly curved back- 

 wards on the dorsal side. The keels of the bridle are very thin, 

 gradually getting finer towards the dorsal side. The tentacle is not 

 bipectinate, but has two rows of delicate pinnules lying close to- 

 gether. The unsegmented fibrous tube has coarse brown rings 

 which do not anastomose and are nowhere split into double 

 rings. 6. S.japonkum (p. 200). 



15 (14) The groove between the protosoma and mesosoma runs near to 



the bridle and curves backwards on the dorsal side. The keels of 

 the bridle are stout, not becoming finer towards the dorsal side. The 

 tentacle is obviously bipectinate, with large curved pinnules. The 

 regular rings, which are sometimes joined by bridges or split into 

 pairs, number seven or rarely six per segment of the segmented 

 tube. 2. S. ekmani (p. 185). 



16 (9) There is a single row of pinnules and no pretentacular groove. 



17 (20) There is a post-tentacular groove near the base of the tentacle. The 



toothed platelets form a single row. 



18 (19) The post-tentacular groove is complete. The pinnules are as long 



as the tentacle is wide. The keels of the bridle are brown. There are 

 two girdles near together and the toothed platelets are 1 1— 12/lx long. 

 The ventral postannular papillae have no cuticular plaques. 



11. S. pusillum (p. 221). 



