SIBOGLINUM 179 



29 (28) The three girdles of toothed platelets all lie close together and the 



platelets are 1 1— 15/lx long. The keels of the bridle are simple. 



30 (31) The glandular girdle behind the bridle is broken both dorsally and 



ventrally, and a long mid-ventral furrow runs in front of the bridle. 

 The toothed platelets, 1 0— 1 2/x long, are arranged in a single row in 

 each of the two or three girdles. The unsegmented tube has close 

 irregular brown rings very variable in length. 25. S. inerme (p. 268). 



31 (30) The glandular girdle behind the bridle is broken only on the dorsal 



side. There is no mid-ventral furrow in front of the bridle. 



32 (33) There is a circular groove behind the base of the tentacle. The 



toothed platelets, 11-12/x long, are arranged in a single row in their 

 girdles. The tube is fibrous, with rings made up solely of thickened 

 fibres. 13. S. microcephalum (p. 227). 



33 (32) There is no circular groove behind the base of the tentacle. The 



toothed platelets, 10-15/x long, are arranged in two to four rows in 

 their girdle. The tube has fibres only in the anterior part and the 

 rather long rings stand out from an homogeneous ground substance. 



21. S. norvegicum (p. 251). 



34 (27) There are two girdles of toothed platelets (three in some indi- 



viduals). 



34a (34b) There are always two girdles of toothed platelets. The front edge of 

 the mesosoma has a slight cutaneous thickening, making a sort of 

 girdle. The toothed platelets, 9-1 2/x long, are arranged in a single 

 row. The segmented tube has regular brown rings, sometimes 

 anastomosing. 18. S. variabile (p. 241). 



34b (34a) There are usually two, occasionally three girdles of toothed 

 platelets close together. The toothed platelets, 10-1 2^ long, are 

 arranged in a single row. The unsegmented tube has brown rings 

 very variable in length. 25. S. inerme (p. 268). 



35 (26) There is no glandular structure behind the bridle. 



36 (37) There is a circular post-tentacular groove. The keels of the bridle 



are in one piece, unbroken. There are three girdles of toothed 

 platelets, with the front two lying some distance from the hindmost. 

 The toothed platelets, 13-14/u, long, are arranged in two rows. The 

 unsegmented tube is fibrous, with simple or split (double) rings. 



16. -S. hyperboreum (p. 236). 



37 (36) A post-tentacular groove is lacking. The keels of the bridle are 



broken, i.e. formed of many isolated pieces. The toothed platelets, 

 9-1 2/x long are arranged in a single row in each of the two widely 



