DISTRIBUTION IN TIME. 181 



ill the living liydroid, and tliese I believe will he found in the leaflets which compose the corbidii; 

 in Jf/laophenia. (Sec woodcut, fig. 30, p. 60.) 



The two rows, then, of appendages in the graptolite would, according to this view, represent 

 a corbula, and the gonangia, if such had existed, would have been borne upon the front of the 

 graptolite along the bases of the appendages. We should hardly, however, expect to find any 

 remains of gonangia in the fossil, for in all living hydroids which have their gonangia protected 

 by corbulse these gonangia are as delicate and perishable as the naked generative sacs in the 

 Gymnohladea. 



The corbulff! of the graptolites were probably open ones, like those of the living Aglaojihenia 

 viyriophyllum, and of several species from extra-European seas, a condition which indicates a low 

 stage of diflPerentiation, and represents a form through which the closed corbula of Aglaophenia 

 pluma, &c., passes in the course of its development. (See woodcut, fig. 30, a, b, c, d. p. CO.) 



The view here adopted of the nature of these supposed generative capsules in the graptolite 

 receives important support from the fact that in every case where they have been satisfactorily 

 observed the denticles of the graptolite become suppressed in that part of the fossil which carries 

 the appendages,^ a fact quite in accordance with what we know of the corbulse in the living 

 hydroids, for in these the hydrothecae with their accompanying iiematophores are replaced by 

 the leaflets of the corbula, while the naked gonangia of other hydroids are never accompanied 

 by an atrophy or other alteration of the hydrothecae or neighbouring parts. 



In both the American and British specimens the appendages in question seem to have been 

 supported by a framework of branched chitinous filaments which remain behind after the destruc- 

 tion of the intervening membrane. The existence of these filaments probably depends on the same 

 morphological conditions as those which determine the presence of the chitinous axial rod, and it 

 must be admitted that we have no known analogy for them in any living hydroid, unless the 

 internal narrow chitinous lamina which passes like a midrib through the corbula-leaflet (woodcut, 

 fig. 30, A, e) admits of being compared with them. 



This comparison of the appendages of Hall with the corbula-leaflets of an Aglaoplienia is in 

 harmony with the view here advocated as to the nature of the denticles of the graptolite which 

 we have compared to the nematophores of an Aglaophenia. I believe the corbulae of the living 

 AglnopUenice to consist essentially of a special and excessive development of the nematophores, so 

 that the graptolite not only in its trophosorae but also in its gonosome would thus present us 

 with an instance of the great development of the nematophoral system at the expense of the 

 hydranthal.^ 



' Hall notices a case {loc. cit., p. 33, pi. b, fig. 9) which he regards as one in wliich the appendages 

 are present in a graptolite which still retains its denticles. Tiiis, however, is by no means a well-marked 

 instance, and one might be permitted to doubt the identity of the structures here figured with the 

 appendages previously described by him. 



' This view of the morphology of the corbulse seems placed beyond doubt by their formation in an 

 undescribed Aglaophenia from the deep-sea dredgings of the United States Coast Survey. The leaflets 

 which form the walls of the large and beautiful open corbulie of this hydroid are mainly composed of 

 the greatly enlarged and transformed nematophores which in the unaltered ramulus lie in front of the 

 hydrothecse. The hydrotheca; of the parts which become transformed into corbul» are not here actually 

 suppressed, but remain of somewhat smaller size, affording the clue to the morphology of the entire organ ; 

 and it can be plainly seen that it is the mesial nematophore of each of these arrested hydrothecae, which 



