STUDY OF THE GRAPTOLITES. 

 Fig. 32. Fig. 33. 



205 



Graptolithus prtstis: enlarged. Graptolithus whitfieldi: enlarged. 



In Retiolites and Retiograptus, the full development of the cellules 

 takes place below the middle of the length of the stipes, while they are 

 less developed towards either extremity. In some species of Retiolites, 

 including one from the Clinton formation, the cellules acquire their greatest 

 development near the base, and the margins are essentially parallel for the 

 greater part of their length. 



In Phyllogra'ptus, the lesser development of the cellules at the base of 

 the stipe is a marked feature. They increase rapidly towards the middle ; 

 and their greatest development is sometimes above and sometimes below 

 the middle, but in all cases becomes rapidly less towards the apex. 



In Dendrograptus, where we have a stout stem without cellules, the 

 branches usually begin at some distance ^above the base, and in their 

 lower part they have scarcely the appearance of being celluliferous : in 

 the middle of their extent the cellules become more distinct, and, so far 

 as can be observed, they are less developed towards the extremities. 



VI. — MODE OF EXISTENCE. 



The numerous individuals of entire or nearly entire fronds illustrated 

 in this memoir, as well as large numbers of others examined, serve to give 

 a pretty clear idea of the general form of the true Graptolites, as well as 

 of their congeners of the same family. Notwithstanding the presence of 

 the radicle or initial point observable in so many species, it does not afford 

 evidence of attachment to the sea-bottom or to any other substance, at 



