ISO MANUAL OF ZOOLOGY. 



zone. All the other Hydrozoa, without a known exception, 

 are marine in their habits. The fixed forms viz,, the Cory- 

 nida^ Sertularida, and Campanularida are represented more or 

 less abundantly in almost all seas, extending from the littoral 

 zone to considerable depths. The oceanic Hydrozoa (Calyco- 

 phoridcR and Physophorida) 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. Of the 

 Hydrocorallincz, Millepora is found in shallow water in the 

 coral-reefs of the West Indies and Pacific Ocean, and the 

 Stylasterids are almost cosmopolitan, the species ranging from 

 the neighbourhood of the coast-line to great depths in the 

 ocean. 



II. DISTRIBUTION OF HYDROZOA IN TIME. The fine-grained litho- 

 graphic slates of Solenhofen and Eichstadt have yielded impressions of 

 Medusa belonging to the existing families of the ALquorida and Trachyne- 

 midcz ; .and the Lucernarida are represented by an ancient form of the 

 Rhizostomida in the same formation. With these exceptions, however, 

 there are few fossil remains which would universally be conceded to be 

 of a Hydrozoal nature. The Oldhamia of the Cambrian rocks of Ire- 

 land 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 hydrothecge. 



The occurrence of Corynida in a fossil condition, except in a few cases, 

 can hardly be said to be free from doubt. Remains possibly 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 supposed Corynid called Palceocoryne has been described 

 from the Carboniferous rocks of Scotland. Species of Hydractinia have 

 also been described from the Cretaceous, Miocene, and Pliocene deposits. 

 The Sertularida and Campanularida are not certainly known to occur in 

 a fossil condition. The fossils called Dendrograptns, Callograptus, Ptilo- 

 graplus, 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 

 Graptolitidcz 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 

 Fig. ^.Didymograptus V-fracttts. the Silurian rocks. The Grapto- 



lites may therefore be regarded as 



characteristic fossils of the Silurian period. Secondly, the diprionidian 

 Graptolites, or those with a row of cellules on each side (e.g., Diplograptus 

 and Climacograptus] , have in Bohemia alone been certainly shown to occur 

 above the horizon of the Lower Silurian rocks. The common genus Didy- 

 mograptus (comprising the "twin" Graptolites, fig. 67), is still more char- 



