180 DISTRIBUTION. 



Whether the denticles of the graptolites gave lodgment to hydranths, or were filled with simple 

 protoplasm, as we know to be the case with the nematophores of the living Plumularida, it is of 

 course impossible to say. If we give analogy its full weight, and extend the resemblance between 

 the denticles of the graptolites and the nematophores of the plumularidans to the nature of their 

 contents, we should then have lodged in the graptolite-denticles, not hydranths, but simple masses 

 of protoplasm, capable of emitting pseudopodial prolongations on which would devolve the duties 

 of conveying nutriment to the colony. The graptolites would thus manifest relations, not merely 

 to the IIydroida, but would exhibit others at least as strong to the Rhizopoda. Indeed, but a 

 step would be needed to convert such an organism into a true rhizopod, which might then be 

 compared to an association of such rhizopodal foruis as Gromia, united into a composite colony by 

 a common tube filled with a common mass of protoplasm. 



A very general feature in the mode of growth of graptolites is found in the fact that while 

 the entire graptolite continues to increase in length, the denticles which are situated towards the 

 proximal end remain of smaller size than those which succeed them, while after thus increasing in 

 size they again often diminish towards the distal end, the broadest part of the graptolite being 

 consequently in this case towards the middle. 



Now, I know of nothing like this among the living Hydroida, while, on the other hand, the 

 nematophores vary in form in one and the same colony, and are sometimes found more or less 

 arrested or otherwise modified towards the proximal end of the branch. 



In support of the hydroid nature of graptolites, the occurrence of generative capsules in 

 these fossils has been recently adduced, and as this is a matter of great importance in the 

 present question we shall here consider the evidence on which it rests. 



Hall has described and figured in one of the double-rowed graptolites [Diplof/rapfus) certain 

 appendages of an irregularly triangular shape, having one angle continued into a narrow band 

 by which they become attached to the body of the graptolite. They are arranged with considerable 

 regularity in two opposite rows, which extend for some length along the sides of the graptolite. 

 These appendages are compared by Hall to the gonangia of a calyptoblastic hydroid.' 



I am indebted to Mr. Etheridge for an opportunity of examining a British specimen of a 

 Bijjiograptus which carries bodies of undoubtedly the same nature as those of Hall, and to Mr. J. 

 Hopkinson, who had previously examined this specimen and determined its nature, for the inspec- 

 tion of an excellent enlarged drawing of it intended for publication." Now, after a full considera- 

 tion of Hall's description and a careful examination of Mr. Etheridge's specimen, while I admit 

 the probability of the appendages in question belonging to the generative system, I am unable to 

 satisfy myself that they are the remains of gonangia. Indeed, they do not appear to me to be 

 capsular bodies at all, but rather hollow laminae, though the way in which they are occasionally 

 folded over on themselves may give them the deceptive appearance of having been capsules. 



The regularity of their disposition, and the close resemblance between those of the American 

 specimens and those of the British, will not allow us to regard them as mere parasitical or accidental 

 growths, and I believe that their connection with the gonosomal system of the graptolite may be 

 considered as probable. If so, then it remains for us to determine the parts which represent them 



' Hall, 'Graptolites of the Quebec Group,' p. 33, pi. is, figs. G — 11. 



° Mr. Hopkiusou's description of the specimen hiis since buen publislicd. See ' Ann. Nat. Hist.' 

 for .Mi.y, 1871. 



