FOSSIL HYDROZOA. 



167 



the balance of evidence is unquestionably in favour of a reference of the 

 Graptolitidce to the Hydrozoa. 



Fig. 59. Didymograptus V-fractus. 

 Lower Silurian (Skiddaw Slates). 



Two leading types may be distinguished amongst the 

 Graptolites, which are termed respectively " monoprionidian " 

 and " diprionidian." The monoprionidian Graptolites, such as 

 Monograptus priodon (fig. 54), are distinguished by the fact 

 that the polypary, whether simple or branched, possesses but 

 a single row of cellules or " hydrothecse." In the diprio- 

 nidian forms, on the other hand, as in Diplograptus (fig. 63), 

 the polypary possesses a row of cellules on each side. It is 

 noticeable that the diprionidian Graptolites, with hardly 

 an exception, are confined to 

 the Lower Silurian rocks ; 

 whilst the monoprionidian 

 forms range from the base 

 of the Silurian to the summit 

 of the Upper Silurian series. 



Numerous genera of Grap- 

 tolites, as here restricted, are know T n to science ; but it 

 will be sufficient to give the diagnostic characters of a 

 few of the commonest and 

 more important types. In the 

 genus Monograptus (figs. 54, 

 55), the polypary is simple, 

 linear, possessing but a single 

 row of cellules on one side, 

 and commencing by an atten- 

 uated, usually curved, base. 

 Species of this genus are found 

 from near the base of the 

 Lower Silurian series to the 

 very summit of the Upper 

 Silurian deposits. 



In the genus Didymograptus 

 (fig. 59), the polypary consists 

 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 



Fig. 60. Tetragraptus quadribrachiatHii 

 (after Hall). Lower Silurian (Skiddaw and 

 Quebec groups). 



