58 HYDROZOA. GRAPTOLITOIDEA 



radiating branches their proximal parts are sometimes 

 enclosed in a horny sheath, termed the central disc, as in 

 some species of Tetragraptus (fig. 14). In those genera 

 which have two branches (fig. 17), the angle between the 

 two is termed the angle of divergence ; it is measured 

 from the hydrothecal side of the dorsal wall of each branch. 

 In some cases {e.g. Monograptus, fig. 13 c) the polypary 

 possesses only a single row of hydrothecse, such forms are 

 said to be uniserial; others (e.g. Diplograptus, fig. 13 d) 

 possess two rows on opposite sides of the polypary — these 

 are the biserial forms, and they may have a single common 



Fig. 14. Tetragraptus headi, Arenig Rocks, a, central disc, x i. 



canal as in Retiolites, or there may be two canals separated 

 by a septum, as in Climacograptus : in many forms of 

 Diplograptus there is only one common canal, but others 

 possess an incomplete septum which, to some extent, 

 divides the canal into two parts. In Dicranograptus the 

 proximal part of the polypary is biserial, whilst the distal 

 part consists of two uniserial branches. In Dimorpho- 

 graptus, on the other hand, the proximal part is uniserial 

 and the distal part biserial ; this genus therefore serves 

 to connect Diplograptus and Monograptus. 



The hydrothecse vary considerably in form in different 



