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 " hydrothecae." 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 Lou>er 

 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. 32 TetrnrrafiSMS quadribrachia- 

 Fig. 31. Didymografisus V-fractus. tus (after Hall). Upper Cambrian (Skid- 

 Upper Cambrian (Skiddaw Slates). daw and Quebec groups). 



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



