DISTRIBUTION IN TIME. 179 



It has been already said that the advocates of the hydroid nature of graptolitcs regard their 

 denticles or hollow lateral offsets as hydrothecac. If this be really the nature of the denticles, the 

 mode in which their cavity opens into that of the main tube is exceptional, for in the living hydroids 

 the point of communication between the hydrotheca and tube of the hydrocaulus is more or less 

 constricted, or even provided with an imperfect diaphragm, so that the hydrothecae become proper 

 chambers, completely differentiated from the common perisarcal tube. Now, the denticles of the 

 graptolite have their cavity uninterruptedly continuous with that of the main tube, there being 

 no diaphragm or constriction of any kind at the point where the one passes into the other.' 



There is, however, another view of the denticles which will meet this difficulty. The re- 

 markable bodies known as nematophores, and which are characteristic of the Plamularklm, have 

 been already described. Among these nematophores there is one form which consists of simple 

 chitinous offsets from the main tube of the hydroid filled with the protoplasmic matter which con- 

 stitutes the characteristic contents of all the nematophores. The mesial and lateral nematophores 

 of Aglaoplienia are of this nature, and a comparison of them with the denticles of a graptolite will 

 show how complete is the resemblance, as may be seen by a reference to woodcut fig. 51, p. 

 117, which represents the nematophores connected with the hydm[\\eca of J///aop/ienia jjl/fma. 

 It has been already shown that the tooth-like processes which project from the edges of the hollow 

 leaflets, which form the walls of the corbula in A(jJaoplienia, are bodies of an entirely similar kind, 

 and the resemblance between these and the tooth-like processes of many graptolites is complete 

 (see woodcut, fig. 30/ p. 60). 



Now, it is not alone in general form that the nematophores of Aijlaophenia resemble the 

 denticles of a graptolite. The mode in which their chitinous sheaths are seen to open into the 

 common canal of the perisarc after the destruction of all the soft parts, is entirely similar to the 

 mode of comminiication between the denticles and the common canal in the fossil — in those cases 

 at least in which the graptolite has afforded facilities for examination such as to leave no doubt as 

 to the structure of the parts in question — and quite different from that in which the proximal 

 extremity of the hydrotheca is connected with the common tube of the chitinous perisarc in the 

 existing hydroid. 



I cannot help believing that this is the true view to take of the morphology of graptolites. 

 If so, the graptolites would admit of an approximation through an luiexpected channel with the 

 Plumularida. They would then be morphologically plumularidans in which the development of 

 hydrothecae had been suppressed by the great development of the nematophores, probably the 

 mesial ones ; while, on the other hand, the existing plumularidan, with well-developed hydrocecae, 

 would present in its nematophores the last traces of the structure of its ancient representative, 

 the graptolite.^ 



' ]\rCoy (' Brit. Pal. Foss.') speaks of a septum at the base of the deulicles in certain graptolites, 

 but subsequent observations have not tended to confirm this statement. 



^ Among the undescribed species in the collections of the United States Coast Survey is a 

 plumularidan in which that part of the stem which lies at the proximal side of the pinna-bearing 

 portion — and is accordingly destitute of hydrothecae — carries along its length a single row of fixed 

 nematophores separated from one another by regular intervals, and appearing to take the place of 

 hydrothecse. This part of the hydroid, if detached from the pinnate portion, might — except for the 

 much greater thickness of the common canal in the living thnn in the extinct form — almost be taken 

 for a recent Rastriics. 



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