334 PART II. SYSTEMATIC ACCOUNT 



5 (4) The postannular papillae are solitary. The front edges of the 



plaques of the metameric section are slightly serrated. The toothed 

 platelets are up to 20/x long. The tube is segmented, with obvious 

 funnels on the front rims of the segments; the regular rings are all 

 equally short. 6. H. celebensis (p. 354). 



6 (3) There are two secondary mesosomal grooves. The cuticular plaques 



on the metameric papillae are about 30/x across. There are about 15 

 papillae in disorderly confusion in the zone of thickened papillae. 

 The toothed platelets are 11— 12yu. long. The rings at the front end 

 of the tube are very short, but in the middle part of the tube they 

 become very irregular in length. 8. H. canadensis (p. 365). 



7 (2) The ventral ends of the keels of the bridle do not meet. There are 



more than ten tentacles. There are two secondary mesosomal grooves. 



8 (9) The keels of the bridle look like combs, consisting of a long 



cuticular axis down the front edge and a broad membrane behind in 

 which dark rod-like thickenings, like the teeth of a comb, are em- 

 bedded. There are about 13 pairs of metameric papillae. The plaques 

 are 30-40/u, across and the toothed platelets up to 18yu long. The tube 

 has short, sometimes irregular rings. 7. H. ctenophora (p. 360). 



9 (8) The keels of the bridle are simple. There are 24-26 pairs of 



metameric papillae. The plaques are 25-35^ across and the toothed 

 platelets up to 14/x long. The tube has regular rings which are not 

 very long. 4. H. beringensis (p. 347). 



10 (1) There are no secondary annular rings on the mesosoma in front of 



the bridle. 



11 (12) The plaques of the metameric papillae are round, about 20/la in 



diameter. The dorsal ends of the keels of the bridle do not meet. 

 The tube has irregular rings with no fibrous external layer. 



1. H. abyssicola (p. 335). 



12 (11) The plaques of the metameric papillae are oval and about 20-40/x 



across. The keels of the bridle fuse dorsally. 



13 (14) The keels of the bridle fuse ventrally. There are less than ten 



tentacles. The plaques of the metameric papillae are about 20/x 

 across. The tube has irregular rings and an external fibrous layer. 



3. H. subtilis (p. 342). 



14 (13) The keels of the bridle do not meet ventrally. There are more than 



ten tentacles. The plaques of the metameric papillae are 30-40/a 

 across. The tube has more or less regular rings, but lacks a fibrous 

 external layer. 5. H. talboti (p. 350). 



