168 CCELENTERATA. 



in the Quebec group of Canada and Skiddaw Slates of Eng- 

 land, and is well represented in the succeeding portions of 

 the Lower Silurian (Llandeilo rocks) ; but no species of the 

 genus is known as late as the Upper Silurian period. 



In the genus Tetragraptus (fig. 60), the polypary consists 

 of four simple monoprionidian branches, springing from a 



central non-celluliferous con- 

 necting process, which bifur- 

 cates at each end. The cel- 

 luliferous branches do not 

 subdivide, and the base may 

 be enveloped in a peculiar 

 corneous "disc," as will be 

 immediately described in the 

 genus Dichograptus. The 

 species of Tetragraptus are 

 exclusively confined to the 

 Skiddaw and Quebec groups 

 (Lower Silurian). 



In the genus Dichograp- 

 tus there are more than four 

 usually eight) simple mono- 

 prionidian branches, which 

 arise from the same number 

 of divisions of a non-cellu- 



Fig. 6l.DichograptiisoctobracMatus, showing the central disc (after Hall). 

 Skiddaw and Quebec groups. 



liferous basal process. In many cases the divisions of the 

 basal connecting process (fig. 61), are enveloped in a species 

 of corneous " disc " or plate, which is believed to have been 

 composed of two laminae. The functions of this disc are 

 doubtful ; but it has been compared with the " float " or 

 buoy of the PJiysophoridce, an order of the Oceanic Hydrozoa. 



