THE HYDROJDS OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 53 



margin of such an interpolated joint is always abrupt, and the perisarc of the existing 

 calycle is not directly continuous with this margin, but is found to merge with that of 

 the interpolated joint some distance within the joint (see fig. 16). From this it would 

 appear that the joints are not all contemporaneous, but represent the remains of former 

 cups which, broken off by accident, have been replaced once, twice, or oftener by the 

 regenerative power of the ccenosarc. In this case, therefore, they can be of no diagnostic 

 value. Evident knobs of chitin are present on the inner surface of the adcauline wall 

 at the base, and of the abcauline wall at the margin. The former do not seem to be 

 reproduced in regenerated hydrothecse. 



The hydrothecse are much more deep than broad, and are characterised by an almost 

 straight abcauline and a strongly convex adcauline contour. They are in most cases 

 considerably longer than the stem internodes. The margin is divided into three pro- 



a. b. 



FIG. 1. Thyroscyp/tus tridentatus. x 70. (a) Primary hydrotheca showing &, and & 2 , knobs of chitin; pp, points on 

 hydrotheca to which the hydranth is moored by comosarcal strands, (b) Internode process and regenerated hydrotheca ; 

 I; knob of chitin marking base of primary hydrotheca j ?, wall of original hydrotheca ; w lt wall of first regenerated 

 hydrotheca ; w.,, wall of second regenerated hydrotheca. 



nounced teeth, separated by three deep and graceful bays, and is furnished with a three- 

 flapped operculum. No trace of a gonosome could be found. 



The following measurements give some indication of the variable proportions of our 

 specimens : 



Stem, length ...... Up to 13 mm. 



Internode, length ..... 0'49*-0'91 mm. 



breadth ..... 0-13-O22 mm. 



Hydrotheca, length ..... Q-60-0'63 



breadth (maximum) . . . Q-22-0'28 



Locality. Gough Island. Depth, 25 fathoms. Bottom, rock. 23rd April 1904. 



Distribution. The species was originally described by BALE from material obtained 

 at Port Phillip, in the south of Australia, as Campanularia tridentata. Subsequently 

 it has been recorded by Professor HARTLAUB (1901) from French Pass, north of South 

 Island, New Zealand. These, so far as I am aware, constitute the only records of the 

 species, so that the Scotia specimens from Gough Island, midway between Cape of Good 

 Hope and Cape Horn, extend its known range from the South Pacific to the South Atlantic. 



* A regenerated internode which succeeded an old truncation of the .stem. 

 (ROY. soc. EDIN. TRANS., VOL. XLVIL, 75.) 



