82 



CCELENTERATA. 



tolites, which are termed respectively " monoprionidian " and 



" diprionidian." The monoprionidian graptolites, such as G. 



priodon (fig. 28), are distinguished by the fact that the polypary, 



Fig. 30. Supposed "Ovarian Capsules" or reproductive buds of Graptolites. 



whether simple or branched, possesses but a single row of cel- 

 lules or " hydro thecae." In the diprionidian forms, on the other 

 hand, as in Diplograpsus (fig. 35), the polypary possesses a row 

 of cellules on each side. It is noticeable that the diprionidian 

 graptolites, with rare exceptions, are confined to the Lower 

 Silurian and Cambrian Rocks ; whilst the monoprionidian forms 

 range from the Cambrian to the summit of the Upper Silurian 

 series. 



At least sixteen genera of Graptolites, as here restricted, are 

 known to science ; but it will be 

 sufficient to give the diagnostic 

 characters of a few of the com- 

 monest and more important 

 types. In the genus Graptolites 

 (figs. 28, 29), the polypary is 

 simple, linear, possessing but a 

 single row of cellules on one 

 side, and commencing by an at- 

 tenuated, usually curved, base. 



Fig. 31. Didymograpsus V-fractus. 

 Upper Cambrian (Skiddaw Slates). 



Fig. 32 Tetragrafisus quadribracJiia- 

 tus (after Hall). Upper Cambrian (Skid- 

 daw and Quebec groups). 



Species of this genus are found from near the base of the 



