HISTORY OF RESEARCH. cxxxv 



virgulate and non-virgulate forms. The Cladophora or non-virgulate forms, like 

 the modern Sertularians and their allies, must have been fixed to rocks in the 

 shallow parts of the sea-shore, and therefore stationary, or to floating objects of a 

 comparatively large size. The Rhabdophora or virgula-bearing Graptolites, on 

 the other hand, were attached to floating sea- weeds, and were therefore drifted far 

 and wide over the waters of the sea at the mercy of winds and currents. The non- 

 virgulate forms grew vertically upwards, and like their modern representatives, 

 were more or less tree-like. The virgulate forms hung vertically downwards, 

 being pendent to the under side of the sea-weed by a thread-like fibre, which in its 

 earliest stages constituted the " nema " proceeding from the apex of the sicula, and 

 which, in the later forms of the Rhabdophora, growing with the general growth of 

 the rhabdosome, constituted the " solid axis or virgula." In other words the 

 Rhabdophora form a special section of the Graptolites, modified for a pseudo- 

 planktonic mode of existence. The modification commenced in later Cambrian 

 times, within the limits of the genus Dictyonema. Some forms of this genus are 

 provided with a short stem and a disc of attachment, and some examples, even of 

 the same species, may have grown vertically, while others may have assumed a 

 pendent position. Abundant examples, however, are met with in which the stem 

 is lengthened out into a long, thread-like hydrocaulus or nema. In these forms 

 the pendent mode of attachment is the only one possible. In harmony with this 

 we find that once this change from dendroid to pendent is initiated, the Graptolites 

 become world-wide in their distribution and remarkable for their abundance. 



In the successive stages of the evolution of the Rhabdophora in time, the 

 number of branches is gradually reduced, and they become turned more and more 

 backwards and upwards towards the light. A first stage is typified by the oldest 

 family (the Dichograptidas), in which the nema is lengthened, and within the limits 

 of which the branches bearing thecas, originally turned downwards owing to their 

 pendent position, turn in the successive genera backwards and upwards towards 

 the line of the nema. The angle of divergence of the branches gradually 

 increases thus from 0° to 360°, and in the Phyllograptidge the branches, which by 

 this time have been reduced to four in number, attach themselves to each other 

 dorsally and grow backwards up the line of the nema, and the thecse have 

 practically recovered their upward direction. 



In a succeeding stage (the Diplograptidse) the branches are reduced to two in 

 number, and the nema, which apparently lengthens with the growth of the 

 organism, has become a typical virgula. 



In the final stage (the Monograptidae) the branches are typically reduced to one, 

 and the evolutionary series is closed. 



It is pointed out that difficulties exist, especially as regards the Dicello- 

 grapta (Leptograptidae and Dicranograptidge), but if it be accepted, even as a 

 broad generalisation, that the typical nema- and virgula-bearing Rhabdophora were 



