xlvi INTRODUCTION. 



fauna. This is recognized in the absence of many fine 

 species of Hydroida which occur on the southern and 

 western coasts, as well as in the presence of a few northern 

 forms that are not found elsewhere, and the prevalence of 

 others which become rare in warmer districts. Thuiaria 

 thuja, which is abundant in the extreme north (North Cape 

 Sec.), is one of the characteristic hydroids of the east coast 

 of England. It all but disappears in the west, being ex- 

 tremely rare along the coasts of Devon and Cornwall. The 

 Aglaophenice, 011 the other hand, which flourish so re- 

 markably in the south-west and along the north-western 

 coasts that are exposed to Gulf-stream influences, barely 

 put in an appearance in the north-east. 



Of A. pluma Mr. Alder mentions only a single specimen 

 as having occurred in Northumberland and Durham. A. 

 myriophyllum is reported equally rare, while A. pennatula 

 and A. tubulifera are absent altogether. 



Three species, Diphasia alata, Calycella fastigiata, and 

 Afflaophenia tubulifera, have been found in Cornwall and 

 also in Shetland, the Hebrides, and on the west coast of 

 Scotland, but nowhere else in Britain. The last-named 

 flourishes luxuriantly in Oban Bay, having for its com- 

 panion there, as in Cornwall, the beautiful coral, Caryo- 

 phylla Smithii; and it has lately been obtained by Mr. 

 Norman in the Hebrides. There can be little doubt that 

 the peculiar distribution of these species, no less than that 

 of the Madrepore, is due to the influence of the warm 

 current, which after bathing the south-western shores of 

 England, sweeps away to the north, touching the Orkneys 

 and Shetland in its course. Afflaophenia tubulifera is also 

 a South-African form. 



To the same genial influence we owe the remarkable 



