liv BRITISH GRAPTOLITES. 



and lateral spines to be distinct from the sac." This sac " extends itself as the 

 common body in its canal along the axis, and gives origin to the budding which 

 develops the successive cellules." He calls attention to the long axis and to an 

 " extension of the common body along the axis above the incipient cells." 



Although these " germs " were observed by Hall only in Diplograptid forms, he 

 considers that they probably differed but little in other species ; in the branching 

 forms the only difference would be that the common body would divide into 2, 4, 8, 

 etc., divisions, each one bearing its axis and common canal. As respects the 

 development of the cellules in general, Hall calls attention to their invariable 

 "lesser development towards the base of the stipe," although the same is often the 

 case at the distal end as well. 



Affinities. — He points out that the method of reproduction in the Graptolites 

 thus observed " shows much analogy with the Hydroidea, and would indicate the 

 Sertularians as their nearest analogues." 



Mode of Existence. — In the case of all those Graptolite species having a single 

 row of cellules, and also in the cases of the two- and four-rowed forms, such as 

 Betiolites, Retiograptus, and PhyUograptits, Hall believes that they were " free 

 floating bodies in the Silurian seas." 



With respect to the Dendroid or tree-like forms, however, he holds that there 

 is some evidence indicative of a different mode of existence, and he infers that these 

 were fixed to the sea bottom by a root or bulb-like expansion at their base. 



Distribution of the Graptolites. — The short section dealing with the geological and 

 geographical distribution of the Graptolites must also be briefly referred to in this 

 place. Hall considers that Graptolites came into existence at the time of the deposi- 

 tion of the Potsdam Sandstone, and attained their greatest development at the epoch 

 of the Quebec group. " Several genera are known in the Trenton formation, and a 

 greater development occurs at the period of the Hudson River formation." " In the 

 Clinton group there is one species of Graptolithus and a Eetiolites, while Dictyonema 

 and Inocaulis occur in the Niagara beds. Dictijonema also ranges up into the Upper 

 Helderberg and Hamilton formations." The wide geographical distribution of 

 Graptolites in America is pointed out by Hall, but no attempt is made by him to fix 

 the geological range of the individual species. 



CHAPTER III. 



1866 to 1880. 



Previous to the year 1866 most of the work among the Graptolites had 

 been done by geologists living on the Continent of Europe or in America; but 



