HYDROIDA II 



135 



of the point. In the northern Sertularia species, we find a general tendency towards spiral arrange- 

 ment of the branches, together with an often very marked approach to unilateral arrangement of 

 the hydrothecce. The spirally placed branches are secondarily ramified, often dichotomically, but in 

 Sertularia tenera irregularly pinuately. From this then, it is in reality but a short step to Hydrall- 

 mania. And finally, there is a certain amount of variation in the Sertularia species, with regard to 

 proportion between the two sinuses, which in some species may be characterised as of equal size. We 

 have thus, as the last and most essential distinctive feature, the fact that the adcauline closing mem- 

 brane in Hydrallmania has a free distal part, whereas in Sertularia, this is reserved for the abcauline 

 membrane. It may be doubtful whether these small differences should be regarded as of generic im- 

 portance; for the present, however, I have found it most correct to follow. Levinsen in distinguish- 

 ing between the two genera. 



Hydrallmania falcata (Linne) Hincks. 



1758 Sertularia falcata, Linne, Systema naturae, Ed. 10, p. 810. 



1868 Hydrallmania falcata, Hincks, A History of the British Hydroid Zoophytes, p. 273, pi. 58. 



Colonies with slender, spiral, monosiphonic and segmented main stem, where as a rule each 

 internodium bears a branch. The branches form an open spiral about the stem, and 

 are themselves regularly secondarily ramified pinnately; the main stem has a hydro- 

 theca in the corner at the origin of the branch, or may in rare cases be altogether 

 devoid of hydrothecaa. Branches and branchlets are segmented, the irregular internodia 

 bear on the one side (the upper) a more or less close row of hydrothecae, the opening 

 part of which is turned out alternately to either side from the median plane of the 

 branch. On young pinnate colonies the hydrothecae are imbedded, and alternately 

 placed in two opposite rows on the branches, the broad plane of which stands verti- 

 cally. The hydrothecse are bottle-shaped to tubular, distally often bent, in young colo- 

 nies with biserial arrangement, somewhat imbedded, in older ones with unilateral ar- 

 rangement and almost sessile. The opening margin is armed with two large lateral 

 teeth, the largest sinus is adcauline, and has a closing membrane with free distal part. 



The gonothecse are oval, as a rule somewhat flattened, with a short tubulous Trondhjem fjord, 

 neck; they are quite smooth or irregular and slightly furrowed transversely. 



Fig.LXXlil. 



depth 90 fathoms 7,0 

 124 0,9 



no 



44 5-6 



_ 115120 metres 

 _ 62 



5 

 98 - 



of a young, pin- 

 nate colony of the 

 Strtularfa-type. 

 (X 40). 



