108 MANUAL OF ZOOLOGY. 



siderable depths. The oceanic Hydrozoa, Calycophoridce. and 

 Physophoridce^ 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 an umbellate manner, but exhibiting no traces of 

 hydrothecae. 



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 

 Palaeozoic Rocks. The oldest of these was described by the 

 author some years ago from the Lower Silurian Rocks of 

 Dumfriesshire under the name of Corynoides. More lately a 

 form called Palceocoryne has been described from the Carboni- 

 ferous Rocks of Scotland. 



The Sertularida and Campanularida are not certainly known 

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

 Callograpsus, Ptilograpsus, and Dictyonema, all at present placed 

 amongst the Graptolites, are, however, not improbably truly 

 referable to the Sertularida. 



There can be little doubt but that the large and singular 

 family of the Graptolitida, should really be looked upon as 

 extinct Hydrozoa, though good authorities still place them 

 amongst the Polyzoa. 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 occur 



above the Silurian Rocks. 



The Gra P tolites ma X therefore 

 be regarded as characteristic 

 Nkj/ fossils of the Silurian period. 



Secondly, the diprionidian 



Fig. zT.Didymograpsus V-fractus. GraptolitCS, Or those with 3. 



row of cellules on each side (genera Diplograpsus, Climacograp- 

 sus, and Dicranograpsus\ have never yet been certainly shown 

 to occur above the horizon of the Lower Silurian Rocks. The 

 common genus Didymograpsus (comprising the " twin " Grap- 

 tolites, fig. 27) is still more characteristic of the Lower Silurian 



