DISTRIBUTION IN SPACE. 157 



Northern Hemisphere, liaving been met with in the Nortli Atlantic, tliongh its exact locahty is 

 doubtful, as Lamouroux's specimens were found on floating seaweed. To the genera peculiar to 

 Australia must also be added the genus Lineolaria of Hincks.' 



Again, in a collection containing sixteen species of calyptoblastic hydroids from the Pacific 

 coast of North America, and examined by Busk and myself, besides one undescribed genus, there 

 is not another in the collection which is not also British ; while twelve species determined bv 

 Alexander Agassiz,^ Andrew Murray,' and Trask,' from the same shores, belong all to British 

 genera. 



A collection of twenty-five species of Cali/pioblastea from Soutli Africa, given to me by Mr. 

 Busk for examination, are also all referable to British genera. 



While almost all the generic forms known to us as occurring in other latitudes than our own 

 are thus widely distributed over the globe, this is by no means the case with the species. The 

 species of hydroids are as a rule confined within determinate and limited areas. Thus of the 

 twenty-five species composing the New Zealand collection just mentioned, three only, namely, 

 the cosmopolitan SertulareUa -pol ponias, with its nearly allied Sertidarella Gay'ii, and the equally 

 cosmopolitan Sertidaria jjimila, have been recorded from European seas ; while among the thirty- 

 one species collected by the " Rattlesnake" in Australian seas. Busk has detected only three 

 European species, namely, Sertularia operculata, Lafoea damosa, and Laomedea voliihiUs ; the last 

 of which he regards as doubtful. Of the remaining species of this collection three only, as pointed 

 out by Busk, occur beyond the limits even of the Australian seas, namely, Sertularia elongata, 

 Lamx., which has been also collected in New Zealand, Sertidarella divaricafa, Busk, in the Straits 

 of Magellan, and A(/laoplienia MacgUUvraiji, Busk, in the Philippine Islands. To these, however, 

 must be added the Sertidarella hupinosa. Gray, which is also a New Zealand species, while it 

 has been collected by Dr. Hooker in the Auckland Islands, and an Aglaophenia {Aglaophenia 

 formosa, Busk), obtained from Australia by Dr. Landsborough, and which I have found in the 

 collections from New Zealand and from South Africa ; while Mr. Hincks records as Australian 

 the Plumularia ohllqua of our own seas. 



Among the hydroids referred to above from the Pacific coast of North America, there are 

 only two which can with any probability be regarded as also European. One of these is Sertularia 

 put/iila, found in California by Dr. Scouler, and contained in ilr. Busk's collection. The specimen 

 is destitute of gonangia, but its trophosome is indistinguishable from that of our British form. 

 The other is Lafoea dumom, forming part of the same collection, and also obtained in California. 

 This is also destitute of gonangia, but so are all the European examples of this species hitherto 

 examined. 



Indeed, the limitation of groups of hydroid species to definite areas is strikingly illustrated 

 by the marked difference between the hydroid fauna of Australia and that of New Zealand. 

 Among all the hydroids hitherto examined from these two regions, only three species, Sertularia 

 elongata, Lan.x., Sertulurella hispiiiosa. Gray, and Aglaophenia formosa, Busk, are common 

 to both. Perhaps when a greater number of Australian and New Zealand species shall have 



' Hincks, in 'Ann. Nat. Hist.,' 1861. 



^ Al. Agassiz, ' Illustrated Catal.' 



' A. Jlnrray, ' Ann. Nat. Hist.,' vol. t, .3rcl ser., p. 250. 



* Trask, ' Proc. Cal. Acad.,' 1857. 



