352 BULLETIN OF THE BUREAU OF FISHERIES 



Our experience has been that the medusas of Phialidium are always most nu- 

 merous at the surface or a meter or so down at most, no matter what the precise 

 locality in the gulf or the time of day, a fact well illustrated in the eastern basin 

 on August 13, 1914, at "station 10249, where only a few were taken in the 50- 

 meter, none in the 175-meter haul, though it was very numerous on the surface" 

 (Bigelow, 1917, p. 305). Similarly, the offshore swarms twice encountered in August, 

 1912, were so close to the surface that the deep hauls yielded very few (no doubt 

 caught by the net in its passage down and up through the rich superficial zone), 

 although the surface nets were clogged with them. 



Trachomedus^e 



The TrachomedusiB as a group are oceanic and only one of them is known to enter 

 regularly into the planktonic fauna of the Gulf of Maine. 



Aglanth.a digitale (Fabricius) 



Whether the known representatives of the genus Aglantha represent two species, 

 a large northern with four otocysts (digitale) and a smaller southern (rosea) with eight 

 of these organs, or only one, has been the subject of much discussion. The most 

 recent observations (e. g., Mayer, 1910; Bigelow, 1911, 1913, and 1915) favor the 

 latter view, it having been proved that the older separation, based on the number of 

 otocysts, can not stand. It is still possible, however, that the genus is represented in 

 different seas by more or less definite size varieties, of which the geographic and 

 seasonal relationships are still to be traced. In the following pages all Aglanthas, 

 large and small, are treated as a specific unit because they have not yet been subjected 

 to examination more critical than has been necessary to establish their generic identity. 



Aglantha digitale is circumpolar and boreal-Arctic. In the northeastern North 

 Atlantic and tributaries its known range includes the White Sea, the Arctic Ocean about 

 Spitzbergen, Barents Sea, the Norwegian Sea, and the northern part of the North Sea. 

 It penetrates thence into the Skager-Rak, which is a center of abundance for it (Kramp, 

 1913), and has been found very plentiful in the Bay of Biscay in depths of 50 fathoms 

 or more (Browne, 1906, as " A. rosea"). The collections made by the plankton expedi- 

 tion show that Aglantha is practically universal between Iceland and Greenland; in 

 fact this probably applies to the whole North Atlantic north of the isotherm of 60° 

 surface temperature. Aglantha is the commonest of the smaller medusas in West 

 Greenland waters (Vanhoffen, 1897), and the records for it off the coast of eastern 

 North America include the east coast of Labrador, the east coast of Newfoundland 

 (Bigelow, 1909a), the Grand Banks, and the continental shelf generally along Nova 

 Scotia (Bigelow, 1917, p. 303). It occurs far and wide in the Gulf of Maine, as de- 

 scribed below, and follows the cool water over the continental shelf as far west and south 

 as the mouth of Chesapeake Bay in winter (Bigelow, 1918, p. 38S) ; sparingly to the 

 latitude of Delaware Bay even in summer. The high temperature of the inner edge of 

 the Gulf Stream forms an insurmountable offshore barrier to Aglantha off the Ameri- 

 can littoral, as it does for so many other boreal members of the plankton. 



Although the early development of Aglantha has not yet been traced, it is prob- 

 able that it is direct, like that of its close ally, the genus Aglaura — that is, without 



