182 DISTRIBUTION. 



If these views be accepted we shall have nearly the entire graptolite in those instances 

 in which the appendages of Ilall have been noticed converted into a corbula, a state of things 

 which naturally follows from the simple unbranched form of the fossil. The graptolite has, in 

 fact, become greatly changed in form, and modified for a special reproductive function in a 

 way which reminds us of the so-called fertile fronds of certain ferns as distinguished from the 

 so-called sterile fronds. 



It is true that the great rarity of these peculiarly modified graptolites is opposed to what 

 we know of living hydroids, for among these we are not acquainted with a single trophosome 

 which we are not justified in believing destined at some period of its life to develope a gono- 

 some. A case, however, bearing some analogy to those of the graptolites would be afforded 

 by fossil ferns, for we know how rare a thing it is to find among the vast midtitudes of indi- 

 viduals with which the coal-measures abound specimens bearing fructification. 



While the graptohtes would thus seem to contrast with living hydroids in theii' rarely 

 developing a gonosome, it is interesting to see them contrasting also in another respect, 

 namely, in their free if not floating habit. And here we are reminded of the gulf-weed of the 

 Sargasso Sea, for throughout the thousands of square miles over which the floating meadows 

 of this remarkable plant extends no one has yet succeeded in finding a single specimen in 

 fructification, though the fructification of closely allied species, which grow attached to rocks like 

 ordinai'y seaweeds and like the rooted trophosomes of the hydroids, is w^ell known. 



Certain bodies found associated with graptolites in the Silurian shales of Dumfriesshire have 

 been described by Dr. Nicholson, who regards them as the " ovarian vesicles" of the graptolites, 

 and as proving the hydroid nature of these fossils.' He describes them as " oval, bell-shaped, 

 pyriform, or rounded, provided with a mucro at one extremity, and surrounded entirely by a fili- 

 ibrm border, resembling in texture the axis of a graptolite." They attain a length of nearly half 

 an inch. 



He has found them not only free, but in many cases attached to the graptolite, not, however, 

 to any constant point, for some spring " from the common canal, others from the apex of a 

 cellule, and others from the under surface of a cellule, the last two modes being the most 

 frequent." 



The largest of these capsules which he has seen attached did not measure more than a 

 tenth of an inch in diameter, and Dr. Nicholson believes that at this stage they become detached. 



has become enormously developed and flattened out so as to form the leaflet of the corbula-walls, while 

 at tlie same time it becomes complicated by carrying along one edge a row of small tooth-like 

 nematophores, as in the corbula-leaflet of Aylaophenia pluma, &c. The hydrothecse, with their 

 neniatophores, which in the untransformed ramnlus constitute a single seiies along the front of the 

 raiiiulus, are, in order to form the walls of the corbula, thrown alternately from side to side. 



In the description of the corbula of Aglaoplienia pluma given above (p. 61), I regarded the 

 gonangia as representing the hydrotheca; of the untransformed branch, and as taking the place of these 

 hydrothecffi in the coibnla. I am now compelled to modify this view, and to regard the gonangia as 

 independent structures, just as in those plumularidaus which are destitute of corbulse. If any represen- 

 tative of the hydrolhecic had existed it would have been found, not ou the floor of the corbula, but in 

 its walls. 



^ Nicholson, in ' Geological ^Magazine,' vol. iv, ISC", p. 259, pi. ii. 



