,, , HYDROIDA II 



Diphasia alata Hi neks. 

 1S55 Sertularia alata, Hincks, Notes on British Zoophytes, p. 127, pi. 2. 

 1868 Diphasia alata, Hincks, A History of the British Hydroid Zoophytes, p. 258, pi. 48, fig. 2. 



Upright pinnate colonies with a not very marked, brown, monosiphonic stem, exceptionally 

 polysiphonic in its basal part; the stem and branches are indistinctly segmented. The hydrothecse 

 are arranged in opposite or snbalternate pairs; they may also, in exceptional cases, be alternately set. 

 On the branches, the hydrotheca; exhibit a tendency towards unilateral arrangement on the front of 

 the colony. The branches arise alternately from the stem ; between two successive branches on the 

 same side of the stem there are three hydrothecse, the lowest at the branch corner. The hydrothecse 

 are large, bent to an angle, with a free distal portion of the adcanline wall between one-third and 

 one-half the length of the hydrotheca, about eqnal to the opening diameter. The abcanline wall has 

 at the middle, near the bend, a prominent transverse inner rib; the angle between the branch axis 

 and that of the distal part of the hydrotheca is greater than 6o°. The plane of the hydrotheca aper- 

 ture is almost perpendicular to the branch axis; the margin has a deep adcanline sinus, in which the 

 large opercular plate is attached. 



The gonothecte are small, almost pear-shaped, distally quadrilateral in section, with a short 

 and blunt distal point in each corner, and a small round distallv central opening, but no neck. The 

 female gonothecse develop no brood-chamber; they are as a rule slightly asymmetrical, whereas the 

 males are symmetrical in structure. 



Material : 



"Thor" 3 5°57' N., 5 a iS ' W., depth 740 metres. 



Diphasia alata belongs to the warmer tracts of the eastern part of the Atlantic, and seems 

 there to be mainly restricted to the lower part of the littoral region and the upper portion of the 

 abyssal. It is a rare visitor in the northern waters, where it has been observed off the west coast of 

 Scotland, at the Hebrides, Shetland, and the west coast of Norway from Jseren to a little north of 

 Bergen. It has not, however, been recorded from the Faroe Islands, Iceland or Greenland. 



Gen. Dynamena (Lamouroux). 



Upright colonies with imbedded, bilaterally built hydrothecae. The hydrotheca aperture has 

 two large lateral teeth, between which there is a large abcanline sinus and a smaller adcauline one; 

 the latter is often divided into two by a slightly prominent median tooth. In each main sinus a mem- 

 brane is fixed, so that the closing apparatus consists of two membranes, the abcanline with a free 

 distal part. The polyp has no pronounced blind sack. 



This definition of the genus Dynamena we also find indicated by Kithn (1913 p. 252). L,evin- 

 sen is not disposed to consider the development of a pronounced blind sack as of any systematic 

 importance (1913 p. 286), which view must doubtless be accounted for by his having apparently con- 

 fused this feature in the organisation with casual S-shaped hydranth contractions. Kiihn regards the 



