DISTRIBUTION IN TIME. 177 



There are also some anomalous forms [Retiolifes, PIij/lIo_<jraptus), whose structure has not 

 yet been detcrniined with sufficient certainty to admit of a satisfactory association with tlic true 

 graptolites ; but the essential features in the morphology of the graptolites, as well as their more 

 important modifications, are e.\})resscd in the genera already cited. 



There is sufficient evidence to show that the graptolites were flexible, and that the solid 

 parts, which are all that have come down to us, were of a horny or chitinous consistence. 

 There is also evidence to show that, though some obscure forms {Dendrograjjtus), associated 

 on insufficient grounds with the graptolites, were apparently rooted, the true graptolites were 

 never directly attached to any other bodies, as we know to be the case with the hydroid 

 trophosomes and most of the corals and Folyzoa of the present day. 



We are absolutely ignorant of the original contents of the main tube and of its lateral 

 offsets, and we know just as little of other soft parts which may have accompanied the chitinous 

 skeleton, so that in attempting to assign to the grai)tolites their position in the system of Nature 

 we are driven to analogy by no means close as our sole guide. 



The resemblance of the forms just described to the trophosome of a calyptoblastic hydroid 

 - — scrtnlarian or plumularian— after the disappearance of all the soft parts, is sufficiently obvious. 

 And it is this rescndilance between the fossil graptolite and the recent chitinous skeletons of the 

 scrtularian and plunudarian hydroids, which has induced modern pakuontologists to refer the 

 fossil to the order Hydroida, regarding the lateral offsets as hydrothecae, and the main tube as 

 the chitinous perisarc of the liydrocaulus.^ 



We shall presently consider whether the exact points of contact between the graptolites and 

 hydroids have been indicated in this comparison. 



The fact which most obviously opposes itself to an acce])tance of the hydroid affinities of 

 graptolites is found in the presence of the rod or " solid axis," which constitutes an essential 

 feature in the structure of the graptolite. This rod was apparently of the same chitinous 

 material as that which formed the rest of the firm skeleton of the graptolite. It is frequently 

 continued for some distance beyond the distal or growing end, while its opposite or proximal end 

 usually terminates in a minute spine {" radicle " of Hall), occasionally continued into a long slender 

 filament, like that of the distal end. It grows with the growth of the graptolite, as can be easily 

 proved by following the progress of the graptolite from its younger stages, and it is difficult to 

 explain its increase of length and thickness without regarding it, like the proper perisarc, as an 



' Tiie sei-tularian affinities of tlie graptolites have been strongly insisted on by Hall, wlio has 

 greatly advanced our knowledge of these fossils in his classical work, ' Graptolites of the Quebec 

 Group,' which forms one of the memoirs of the Geological Survey of Canada. On the structure and 

 principal modifications of graptolites, the works of Barrande (' Graptolites de Boheme ') and of Geinitz 

 should also be consulted. The sertularian affinities of graptolites have also been defended by Mr. 

 AVm. Carruthers, of the Botanical Department, British Museum, and I know of no one who has 

 worked out this question with so much care and completeness. See especially his " Revision of the 

 British Graptolites," in the 'Geological Magazine,' vol. v. The same view is advocated by Dr. J. 

 Alleyu Nicholson, who has also paid special attention to the British graptolites, and has contributed 

 some valuable facts to our knowledge of them. 



I must here express my thanks to Mr. Carruthers for the liberal way in which he has placed at 

 my disposal his large collection of graptolites, and for the aid ^^hich I Ijavc derived from his extensive 

 acquaintance with the literature of the subject. 



