4 SYDNEY J. HICKSON AND F. H. GRAVELY. 



has been proposed must present to those who are working upon new materials or on 

 new lines of research many inconsistencies and inconveniences. 



The best classification is, after all, the one that is most convenient for the 

 particular class of investigation that the author is engaged upon, and consequently 

 those who approach the Hydrozoa from one side will be inclined to adopt a classifica- 

 tion which to authors who approach it from another seems erroneous or unsatisfactory. 



It is clearly not to the advantage of Science that the classification of a group of 

 animals should constantly change, and it is better to adopt one which may in some 

 respects seem unsatisfactory than to propose alterations upon any grounds other than 

 those of wide and far-reaching new investigation. 



Allman and Hincks, the two great pioneers of the zoology of Hydrozoa, distributed 

 the genera amongst a large number of families ; in fact, according to their system a 

 great many genera stand alone, or almost alone, in a family. 



The tendency of recent systematists has been to rearrange the genera in such a 

 manner as to reduce the number of families, and this tendency appears to us to be not 

 only more convenient but to be founded on a sound scientific basis. 



The system we have used is that adopted by one of us in his essay on the 

 Coelenterata in the Cambridge Natural History, Vol. I.; a system which, like any other, 

 is open to many criticisms in detail, but has been found on the whole to suit our 

 purpose as well as any other. 



FAMILY BOUGA1NVILLIIDAE. 



This family includes, according to our system, the sub-families Bougainvilliinae 

 (Bougainvilliidae, Gegenbaur), Margelinae (Margelidae, Hoeckel), Dicorynae (Dicory- 

 iitilae, Allman), Eudendriinae (Eudendriidae, Hiucks), and the Bimeriinae (Bimeriidae, 

 Allman). 



The hydranths in this family have a single circle of filiform tentacles, and the base 

 of the hydranths is protected by a tube of perisarc. 



The inclusion of Stylactis in the sub-family Bimeriinae is perhaps the most 

 unsatisfactory feature in this arrangement, but as we have found in some cases a short 

 collar-like tube of perisarc at the base of the hydranths (fig. 33, c. p.), which has not 

 been observed, so far, in any species of the family the Podocorynidae to which in 

 other respects Stylactis has many affinities, we have retained it in this family. 



SUB-FAMILY BOUGAINVILLIINAE. 



PERIGONIMUS ANTARCTICUS. 

 (Plate I., figs. 1, 2, 3 ; and Plate IV., fig. 32.) 

 iinin sp., Hiirthinb, Voy. du Belgica, Hydroidi'ii, (1004). 

 Locality. Common in McMurdo Bay at depths of 0-130 fins. 

 The species was found in no less than eleven of the bottles sent to us, and is usually 



