458 PART II. SYSTEMATIC ACCOUNT 



30 (25) The rings are mud-coloured, or more or less colourless, but not 



white. The walls of the tube are not stout, but dense. 



3 1 (34) The length of the rings is not more than one quarter the diameter 



of the tube, except that divided rings may reach one third the 

 diameter of the tube. 



32 (33) The rings are regular, complete and very short, recalling narrow 



barrel-hoops; their length does not exceed one quarter the diameter 

 of the tube. The interval between successive rings is very large. 



Siboglinum robustum (p. 254). 



33 (32) The rings are often irregular, incomplete or anastomosing with 



one another; their length rarely reaches one third the diameter of 

 the tube which is 0-4-0-6 mm. Siboglinum atlanticum (p. 263). 



34 (31) Most of the rings are longer than one quarter the diameter of the 



tube. 



35 (42) The front part of the tube is segmented. 



36 (37) There are 12-18 rings per segment. The anterior rings in each 



segment anastomose more or less tightly with one another. Along 

 the greater part of the tube the rings come close together and partly 

 fuse to form double rings. The diameter reaches 0-25 mm. 



Siboglinum fedotovi (p. 193). 



37 (36) There are less than 10 rings per segment. 



38 (39) In the hind part of the tube the rings anatomose in pairs by means 



of bridges, or coalesce forming secondary rings, whose length is a 

 little more than half the diameter of the tube. There are usually 

 seven rings per segment, rarely five or six. The diameter of the tube 

 is 0-12-0-16 mm. Siboglinum ekmani (p. 185). 



39 (38) The rings do not anastomose or coalesce. 



40 (41) There are 3-9 rings per segment. The intervals between the rings 



in the front part of the tube are very narrow, like mere chinks. The 

 diameter of the tube is 0-12-0-20 mm. 



Siboglinum variabile (p. 241). 



41 (40) There are 8 rings per segment. The intervals between the rings 



are narrow but not chink-like. The diameter of the tube is 0-13- 

 0-14 mm. Siboglinum pinnulatum (p. 190). 



42 (35) The tube is unsegmented. 



43 (46) The rings split into two or coalesce into double rings, forming 



secondary rings. 



44 (45) The rings have even edges. Adjacent rings are divided by light 



intervals or run into one another. The length of the secondary 



