44 HYDROZOA. GRAPTOLITOIDEA. 



gula is single, and is placed dorsally in the monoprionidian 

 genera, but in the diprionidian it is central, being situated 

 in the middle of the wall separating the two common 

 canals. Occasionally, however, it is double, as in Retio- 

 lites, in which the axes are separated and placed in the 

 wall on opposite sides of the common canal. The position 

 of the sicula also varies. In Monograptus it is united 

 to the dorsal surface of the polypary, the pointed end 

 being directed distally (fig. 8 e, s). In Didymograptus its 

 broad end only is united to the polypary, the pointed 

 end being directed downwards (fig. 8 f, s). In Dicello- 

 graptus, in which there are two branches diverging at 

 a wide angle, it forms a kind of axial spine. In Diplo- 

 graptus it is enclosed by the hydrothecas (fig. 8 g, s). 

 Lastly, in some forms it does not persist in the adult 

 state. In those genera which have two branches, the 

 term angle of divergence is applied to the angle between 

 the hydrothecal margins. 



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 hydrothecse, 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 original material has been replaced by iron pyrites, the 

 natural form of the polypary is preserved. 



The reproduction of the graptolites appears to have 

 been similar to that which takes place in some modern 

 hydrozoans. In a few diprionidian forms (fig. 9) sac-like 

 bodies have been found attached to the sides of the 

 polypary, these appear when perfect to be pear-shaped, 

 and in some cases at any rate, they are connected with 

 the virgula by means of fibres. These bodies resemble 



