GRAPTOLITHINA 



171 



side of the stipe. In Didymograptus (Fig. 127) the second hydrotheca 

 grows across the sicula to open on the opposite side, and the first and 

 second hydrothecae go on budding independently, so that we have a 

 colony with two stipes or branches. By another modification later, 

 hydrothecae bud off two individuals instead of one, and colonies like 

 Tetragraptus (Fig. 128) and Bryograptus are formed. In Diplograptus 

 and Climacograptus (Fig. 126 C) there is a biserial stipe, either formed 

 of two uniserial stipes growing back to back and thus separated by a 



xi. 



Fig. 128. 



Fig. 129. 



Fig. 128. Tetragraptus. Ordovician Rocks, a, central disc. 



Fig. 129. Diplograptus foliaceus from the Utica Slate, New York, x f , 

 After Ruedemann. For description see text. 



median septum, or as a result of alternate budding throughout the 

 colony. 



In the absence of a coenosarc the graptolites were not attached by 

 a creeping hydrorhiza, such as occurs in Calyptoblastea. There was, 

 however, a thread coming off from the end of the sicula which ended 

 in a disc, by which it is supposed that the graptolites were attached 

 to floating seaweed (Fig. 127 B). It is also possible that some grapto- 

 lites were independent planktonic organisms with a pneumatocyst or 

 other kind of float. Such a pneumatocyst appears to be shown in 

 Diplograptus (Fig. 129) as a square central body from which a number 

 of stipes radiate. There is also a circle of round bodies which are 

 possibly gonophores, as they contain siculae. In any case the grapto- 

 lites were true pelagic organisms and their floating habit gave them 



