158 DISTRIBUTION. 



become known, other points of agreement will he fonnd ; bnt it is not likely that the percentage 

 of species common to the two will be much altered ; and this will appear all the more remarkable 

 when we bear in mind that a very large proportion of Australian PoJij'zoa are, as ascertained by 

 Busk, common to that country and New Zealand, 



Even within the region of the British seas a few s])ecies occur which are not only 

 exclusively British, but which are either absolutely confined to the Northern or the Southern shores 

 of the British Isles, or have at least the metropolis of their distribution on one or other of these 

 shores. Among these we may cite, as examples of Northern forms, Syncoryne eximia, Syncoryne 

 pulchella, Etidendriiim unnulatum, Tubularia bellis, Halecium lahrosum, and Sertularia fuscci; 

 while Coryne vnyinata, Periyonhnns serpens, Hydrnntliea wargarica, OjjJnoides mirabilis, and 

 Aylaoplienia permatala may be cited as especially belonging to the Southern shores of our islands. 

 It is not improbable, however, that further observations will show that many species have a much 

 wider range than what our present knowledge would justify us in attributing to them. 



There is, however, one region which affords a remarkable exception to that limitation of 

 species within definite areas which is here insisted on. I have examined a collection of twenty- 

 five species of calyptoblastic hydroids from South Africa in Mr. Busk's possession, and find them 

 not only all referable to British genera, but with no less than ten of the species indistinguishable 

 from hydroids occurring on the British shores. These ten species are Sertularia pumila, Sertu- 

 laria abietina, Sertularella polyzonias, Bipliasia afferiuata, Diphasia pinnala, Antenmdaria 

 antennina, var.,^ JylaopjJienia tubulifera, Filelluni serpens, LaJ'oea pygmea, and Lafom parvula -^ 

 while another British species, Sertularia operculata, has also been recorded by Busk from the 

 same seas. 



Among the ten hydroids, however, which I have thus identified with British species, the 

 specimens referred to Sertularella polyzonias and Bipliasia jjinn at a are the only ones in which the 

 gonosome is present ; but as the trophosomes of the others are indistinguishable from those of the 

 species to which I have referred them, we are justified for the present in assuming the identifica- 

 tions as correct. 



Admitting the correctness of this determination, the proportion of South African species 

 identical with British ones is quite exceptional, and unexpectedly large ; so much so that I have 

 little hesitation in explaining the correspondence between the two faunas by referring it to the 

 transporting agency of the large number of European ships which frequent our South African 



* This Antennularia, though branched, appears to come nearer to Antennularia antennina than to 

 Antenmdaria ramosa. The short intervening iuternode which is characteristic of Antennularia anten- 

 nina is here occasionally present in the ramuli, though more frequently absent ; while the rarauli them- 

 selves agree with those of Antennularia anlennina in being more distant tlian in Antennularia ramosa. 

 The form is plainly intermediate between Antennularia antennina and Antennularia ramosa, and would 

 seem to shake the validity of the latter as a true species. 



^ Though I believe I am right in the determination of these two species of Lafoca, it must not 

 he forgotten that the smaller species of Lafoea are very obscure, and by no means easily distinguished 

 from one another. No gonosome having as yet been found in any of them, we are, in the absence of 

 the hydranths which may possibly afford diagnostic features, forced to characterise them from what are 

 in many cases very slight differences in the form of the liydrothecre — diffcicuces which have scarcely 

 the constancy necessary for a specific cliuracter. 



