341 



such a ])osition as to become reii'iilarly exi)Ose(l for several 

 hours each day during el)l) tide. p]veii the dehcate and 

 unprotected Ghiva of our northern shores dehghts to live 

 under such conditions, and is rarely found in deeper water 

 or in tide pools. Most hydroids, however, can not with- 

 stand such exposure, and hence they are found only in the 

 deeper waters or the deeper tide pools. 



The majority of hydroids are inhabitants of the littoral 

 district, and they usually occur in the more moderate depths. 

 The tubularian hydroids probably never extend to any con- 

 siderable depths, the deep-water forms belonging chiefly to 

 the Plumularidfie.* One of the abyssal Plumularians was 

 obtained ly the "Blake"' at a depth of 1,240 fathoms, which 

 exceeded by more than 300 fathoms that at which Plumul- 

 arians were obtained by the " Challenger." ■;■ 



The Palceozoic class of graptolites is the most inn)ortant 

 group of the Hydrozoa from a stratigraphist's point of view, 

 for it constitutes one of the most important classes of index 

 fossils known. The bionomic characters of this class have 

 been most thoroughly discussed by Professor Lajtworth, 

 a synopsis of whose views will here be given.:!: 



Two distinct groups of Graptolites are generally recog- 

 nized, viz: Chidophora, oi* dendroid graptolites, in which 

 the pol^'parium is more or less tree-like with a distinct 

 hvdrocaulus. and Ehabdophorn, or virgulate gra})tolites, 

 without a hvdrocaulus, but with a horny axis, the virgula, 

 which is prolonged beyond the end of the colony. To the 

 former group belongs Dyctyonema, to the latter the true 

 graptolites (Monograptus, Diplograptus, etc.). The major- 

 ity of the dendroid graptolites undoubtedly grew attached 

 to sea-weeds, rocks, or other sup])orts, in the manner of 

 most modern hydroids, but some were attached to floating 

 algfe, leading a pseudo-planktonic existence. Cases of 

 such attachment have actually been observed among these 

 fossils. 



* Agassiz, "88, 11., 35. 

 t Agassiz, loc. cit. 

 t Walther, '97. 



