60 HYDROZOA. GRAPTOLITOIDEA 



In the uniserial genera the virgula, when present, is 

 placed in the periderm opposite the row of hydrothecae, 

 but in the biserial forms it is central, being situated either 

 in the middle of the common canal, as in some forms of 

 Diplograptus, or in the septum separating the two canals, 

 as in Climacograptus. In several genera (Didymograptus, 

 Phyllograptus, Tetragraptus, Dichograptus, Dicellograptus) 

 the virgula is not found in the wall of the common canal, 

 but projects as a thread from the pointed end of the 

 sicula. 



The position of the sicula varies in different genera. 

 In Monograptus it is united to the dorsal surface of the 

 polypary, the pointed end being . directed distally (fig. 

 13 c, s). In Diplograptus it has a similar position but is 

 more or less completely enclosed between the hydrothecae 

 (fig. 13 d, s). In Didymograptus its broad end only is 

 united to the two branches of the polypary, the pointed 

 end being directed proximally (fig. 17, s). In Dicellograptus 

 it projects like a spine between the two branches. 



The appearance of even the same species of graptolite 

 varies considerably according to its mode of preservation. 

 Frequently it is flattened to a film, and when this is the 

 case we may get a side view, a front view showing the 

 mouths of the hydrothecae, or a back view; in the two 

 latter cases the margins will be parallel. But when the 

 original material has been replaced by iron pyrites, or 

 when the graptolite is preserved in a limestone, the 

 natural form of the polypary is often retained. 



No medusoid form is known in the graptolites; but, 

 in one respect, their mode of reproduction appears to have 

 been similar to that which takes place in some living 

 Hydrozoa. In a few biserial graptolites (fig. 16) sac-like 

 bodies have been found attached to the polypary ; these, 



