FOSSIL HYDROZOA. 



Lower Silurian series to the very summit of the Upper Silurian 

 deposits. 



In the genus Didymograpsus (fig. 31), the polypary con- 

 sists of two simple monoprionidian branches, which spring 

 from a common point, which is almost invariably marked by 

 a small spine-like " radicle." The genus attains its maximum 

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

 land (Upper Cambrian), and is well represented in the earlier 

 portion of the Lower Silurian period (Llandeilo Rocks) ; but 

 no species of the genus is known as late as the Upper Silurian 

 period. 



In the genus Tetragrapsus (fig. 32), the polypary consists of 

 four simple monoprionidian branches, springing from a central 



non-celluliferous connecting 

 process, which bifurcates at 

 each end. The celluliferous 

 branches do not subdivide, 

 and the base may be en- 

 veloped in a peculiar cor- 

 neous "disc," as will be im- 

 mediately described in the 

 genus Dichograpsus. The 

 species of Tetragrapsus are 

 exclusively confined to the 

 Skiddaw and Quebec groups 

 (Upper Cambrian). 



In the genus Dichograp- 

 sus there are more than four 

 (usually eight) simple mono- 

 prionidian branches, which 

 arise from the same number 

 of divisions of a non-cel- 

 luliferous basal process. In 

 many cases the divisions of 

 the basal connecting pro- 

 cess (fig. 33), are enveloped 



Fig. 33- Dichograpsus octobrachiatus, showing the central disc (after Hall). 

 Skiddaw and Quebec groups. 



