HYDROZOA. GRAPTOLIT01DEA. 47 



always curved and with branches given off from the 

 hydrothecal margin. Upper Tarannon to Lower Ludlow. 



Ccenograptus. Polypary bilateral, monoprionidian. 

 The central non-celluliferous portion bent into an S-shape, 

 giving off branches from the convex portions. Hydrothecae 

 in contact only, not overlapping. Sicula median, tri- 

 angular. Upper Llandeilo. 



Didymograptus. (fig. 8 f.) Polypary bilateral, mo- 

 noprionidian, consisting of two simple branches diverging 

 at an angle which varies from to 180 in different 

 species. HydrothecaB rectangular, in contact. The pointed 

 end of the sicula directed downwards. Lower Arenig to 

 Upper Llandeilo. 



Tetragraptus. Polypary bilateral, monoprionidian, 

 consisting of four simple radiating branches. Hydrothecse 

 rectangular, in contact. A central disc may or may not 

 be present. Lower Arenig. 



Dicranograptus. Proximal portion of the polypary 

 diprionidian, but dividing distally into two monoprionidian 

 branches. Terminations of the hydrothecse isolated and 

 incurved. Upper Llandeilo to Lower Bala. 



Dicellograptus. Like Dicranograptus, but mono- 

 prionidian throughout, the two branches united by the 

 sicula only. Angle of divergence greater than 180. 

 Upper Arenig to Upper Bala. 



Diplograptus. (fig. 8 g.) Polypary diprionidian. 

 Hydrothecse overlapping, and placed obliquely. Virgula 

 prolonged beyond the distal and sometimes also the 

 proximal extremity of the polypary. Sicula imbedded. 

 Lower Arenig to Tarannon. 



