378 Prof. AUman on the Morphology 



bulae 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 nema- 

 tophore of each of these arrested hydrothecee which has be- 

 come enormously developed and flattened out so as to form the 

 leaflet of the cor bula- walls, while at the same time it becomes 

 complicated by carrying along one edge a row of small tooth- 

 like nematophores, as in the corbula-leaflet of Aglaophenia 

 pluma &c. The hydrothec^, with their nematophores, which 

 in the untransformed ramulus constitute a single series along 

 the front of the ramulus, are, in order to form the walls of the 

 corbula, thrown alternately from side to side. 



If these views be accepted, we shall have nearly the entire 

 graptolite in those instances in which the appendages of Hall 

 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 develoj)e a gonosome. A case, however, bearing 

 some analogy to that of the graptolites would be afforded by 

 fossil ferns ; for we know how rare a thing it is to find, among 

 the vast multitudes of individuals with which the coal-mea- 

 sures abound, specimens bearing fructification. 



While the graptolites would thus seem to contrast with 

 living hydroids in their 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 mea- 

 dows of this remarkable plant extend, no one has yet succeeded 

 in finding a single specimen in fructification, though the fruc- 

 tification of closely allied species, which grow attached to rocks 

 like ordinary seaweeds and like the rooted trophosomes of the 

 hydroids, is weU known. 



Certain bodies found associated with graptolites in the Silu- 

 rian 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*. 



* Nicliokoii; in 'Geological Magazine,' vol. iv. 1867, p. 259, pi. 2. 



