HYDROIDA II 147 



cation; they are divided into irregular internodia with a large and varying number of hydrothecae on 

 each. The hydrothecae are alternately or subalternately set in two opposite rows on the branches; their 

 symmetrical plane coincides with the broad plane of the branch. The hydrothecae are deeply imbed- 

 ded; the adcauline wall has a free distal part between half and once the opening diameter or between 

 one-eighth and one-fourth the length of the hydrotheca itself. The hydrotheca opening is round, the 

 margin as a rule having a more or less distinct abcauline sinus, so that the hydrotheca aperture often 

 appears furnished with two sligtly prominent teeth. The distance between two hydrothecae in the 

 same row is very slight, less than half the opening diameter. The operculum is formed by a large 

 opercular plate abcaulinally attached. 



The gonothecae are set on the upper side of the branches, and proceed from close under the 

 base of the hydrothecae. They are narrowly pear-shaped, with a short, often almost rudimentary cy- 

 lindrical neck, and broad aperture. 



Material: 



"Ingolf St. 44 6i°42' N., 9°36' W., depth 545 fathoms, 4,8° 

 - - 144 6 2 °49' N., 7°i2' W., - 276 i,6° 



Greenland: Davis Strait, 80 — (without further details). 



The synonymy of this species is extremely difficult to determine; we are to a great extent 

 forced to rely on geographical data. Kirchenpauer (1884) was really the first to point out what 

 particular characters distinguish the two species which have been taken together under the name of 

 Thujaria articulata; to him belongs the credit of having given the definite characterisation of that 

 South Sea form for which he has • • doubtless correctly — reserved the name Thujaria articulata 

 (Pallas). This is specifically quite different from the northern species which has been given the same 

 name, but which should properly be known as Thujaria lonchitis (Ellis et Solander). That the two 

 have hitherto been so generally confused is due partly to the fact that writers have relied upon the 

 records in the extant literature, partly to the frequent inability on the part of investigators to procure 

 material of more than the one species. Thus it happens also, that even Be dot, in his brilliant work: 

 "Materiaux pour servir a l'histoire des Hydroi'des" has committed the error of noting the two species 

 as synonyms, despite the fact that the drawings given by Hincks (1868), and Allman (1888) could 

 hardly be regarded as illustrating the same species. Stechow (1913 p. 152) and Broch (1914 p. 34) 

 have, after further study of both species, given a closer definition of Thujaria articulata, from which, 

 and from the descriptions given of Thujaria lonchitis, it is evident that they differ even more than is 

 usual between other species of the genus in question. 



Thujaria lonchitis appears to be an arctic circumpolar species, which may penetrate into the 

 boreal areas. But as it has constantly been confused with related forms, especially with Thujaria 

 laxa, its geographical character cannot be defined with certainty. From the list of material for 

 the remaining northern species, we see that it has also in the waters here investigated constantly 

 been confused with other Thujaria species, and it will therefore be most correct to leave the final 

 decision as to its biogeographical position in abeyance, pending the acquisition of new and more 



reliable data. 



19* 



