170 DISTRIBUTION. 



The lijdroicls which tenant tlie Abyssal Zone include some higlily interesting forms unknown 

 in the others. Among them are most of the new species discovered by the " Porcupine" explorers. 

 It is an interesting fact, however, in the bathymetrical distriljution of the Hydroida that several 

 species ^hich frequent much higher zones extend downwards into this. Thus Laomedea 

 verticellata has been obtained from the Aljyssal Zone at a depth of 300 fathoms, Sertidarella 

 polyzonias at a depth of 374 fathoms, while IlijdraUmania falcata has been brought up from a 

 depth of 542 fathoms, and Thuiaria articulata from a depth of 632 fathoms. 



The following species have not yet been found in any zone above the Abyssal. 



Lafoea halecioides, Lafoeina tenuis, Diphasia coronifera, Thuiaria Jlexilis, Thuiaria laxa, 

 Thuiaria salicorma, Plumularia ramulifera, Gonocladium plumosum } 



Deep Lacustrine Fauna. — In connection with the bathymetrical distribiition of the 

 Hydroida, the recent researches of Dr. Forel on the deep fauna of the Lake of Geneva must 

 be mentioned. He has found the mud of this lake at a depth of even 300 metres by no means 

 destitute of animal life. The species occurring at this depth belong to the groups of Tnsecta 

 (larvaj), Arachnida, Crustacea, and worms. At a depth, however, of 75 metres (250 feet) a 

 much richer fauna was found, for in addition to the groups represented in the deeper zone there 

 occurred here Mollusca, Ccelenterata and Infusoria, the Ccelenterata being represented by a 

 Jli/dra.' 



2. Distribution in Time.' 



Among the extinct forms which have been referred to the Hydroida are the graptolites of 

 the Silurian period, and though there is much which is still obscure and enigmatical in these 

 remarkable bodies, our present knowledge of them points towards the Hydroida as their 

 probable allies. 



Admitting the hydroid affinities of the graptolites, it is among these remarkable fossils that 

 we must seek for the chief evidence of the existence of Hydroida in former ages of the world, 

 for the association of the rugose and tabulate corals with the Hydroida, which has been insisted 

 on by Agassiz, has not yet received the verification which is necessary for its acceptance, while 

 the number of other fossils which have been on good grounds referred to the Hydroida is very 

 small. 



We shall presently consider the well-established instances of ancient hydroid life, and shall 

 discuss the nature of graptolites and their probable points of contact with the Hydroida ; but 

 it will be best to notice first certain fossils which have, without, as I believe, sufficient evidence, 

 been adduced as former representatives of this order. 



^ The new species here enumerated will be described in my Report on the Hydroida of the 

 " Porcupine" Expedition. 



^ F. A. Forel, " L'Etude de la Fauna profonde du Lac Leman," ' Bull, de la See. Vaudoise de 

 Sci. Nat.,' vol. X, p. 217. 



^ To Mr. Etheridge, Palaeontologist to the Geological Survey of Great Britain, I must here 

 express ray thanks for his kindness in looking over the proofs of this section before they were finally 

 sent to press. 



