*41 



• • 



such a position as to become regularly exposed for several 

 hours each day during ebb tide. Even the delicate and 

 unprotected Clava of our northern shores delights 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 hvdroids probablv never extend to any con- 

 siderable depths, the deep-water forms belonging chiefly to 

 the PlumularidaB.* One of the abyssal Plumularians was 

 obtained by 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." i 



The Pakeozoic class of graptolites is the most important 

 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 Lap worth, 

 a synopsis of whose views will here be given. % 



Two distinct groups of Graptolites are generally recog- 

 nized, viz: Cladophora, or dendroid graptolites, in which 

 the polyparium is more or less tree-like with a distinct 

 hydrocaulus, and Rhabdophora, or virgulate graptolites, 

 without a hydrocaulus, 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 supports, in the manner of 

 most modern hydroids, but some were attached to floating 

 algre, leading a pseudo-planktonic existence. Cases of 

 such attachment have actually been observed among these 

 fossils. 



* Agassiz, '88, n., 35. 

 + Agassiz, loc. cit. 

 X Walther, '97. 



