COiLENTERATA : HYDROZOA. II3 



PhysophoridcR, are chiefly characteristic of tropical, seas ; but 

 they are found also in the Mediterranean, and even in seas not 

 far from, or even within, the Arctic circle. 



II. Distribution of Hydrozoa in Time. — ^With the excep- 

 tion of the impression of a Medusa said to have been observed 

 by Professor Agassiz in the fine-grained lithographic slate of 

 Solenhofen (Oolite), there are no fossil remains which would 

 be universally conceded to be of a Hydrozoal nature. The 

 Oldhamia of the Cambrian rocks of Ireland has, indeed, 

 been regarded as belonging to the Hydrozoa; but it is believed 

 by Mr Salter to be really a plant. It consists of a main 

 stem with numerous secondary branches, springing from the 

 axis in en umbellate manner, but exhibiting no traces of 

 hydrothecas. 



The occurrence of Corynida in a fossil condition can hardly 

 be said to be free from doubt. Remains probably referable to 

 this order have been, however, recently discovered in the Pal- 

 Eeozoic Rocks. The oldest of these was described by the 

 author gome years ago from the Lower Silurian rocks of Dum- 

 friesshire under the name of Corynoides. More lately a form 

 called PalcBocoryne has been described from the Carboniferous 

 rocks of Scotland. 



The Sertularida and Campanularida axe not certainly known 

 to occur in a fossil condition. The fossils called Dendrograpsus, 

 Callograpsus, PUlograpsus, and Dictyonema, all at present placed 

 amongst the Graptolites, are, however, not improbably truly 

 referable to the Seriularida. 



There can be little doubt but that the large and singular 

 family of the Graptolitidcz should really be looked upon as 

 extinct Hydrozoa, though good authorities still place them 

 sxaang^t ^e. Polyzoa„r. As regards their distribution two facts 

 are chiefly noticeable. In the first place, no Graptolite, except 

 the doubtful genus Dictyonema, has hitherto been found to oc- 

 cur above the Silurian rocks. 

 The Graptolites may therefore 

 be regarded as characteristic 

 fossils of the Silurian period. 

 Secondly, the diprionidian 

 Graptolites, or those with a „.„„,, , „ , , 



^ - ,,' , ... Fig. 28. — Dtaymograpsus V-jractus. 



row of cellules on each side 

 (genera Diplograpsus, Climacograpsus, and Dicranograpsus), 

 have never yet been certainly shown to occur above the hori- 

 zon of the Lower Silurian rocks. The common genus Didy- 

 mograpsus (comprising the " twin " Graptolites, fig. 28) is still 

 more characteristic of the Lower Silurian period. In Didymo- 



