74 



CCELENTERATA. 



groups (the Graptolitida and Rugosa) are wholly extinct, whilst 

 the group of the Tabulate Corals is now much reduced in 

 numbers. The above conclusion is further rendered uncertain 

 by the existence in the sub-kingdom of some groups which, 

 from their absence of hard parts, have left no traces of their 

 past existence. 



FOSSIL HYDROZOA. 



Of the living orders of Hydrozoa, the Fresh-water Polypes 

 (Hydrida) and the Oceanic Hydrozoa (Calycophorida and 

 Physophorida) have left no traces of their former presence, as 

 might have been anticipated from their want of hard structures. 

 The order of the Medusida and the sub-class Lucernarida 

 (Jelly-fishes and Sea-blubbers) are equally destitute of hard 

 parts, and their absence from the 

 palaeontological record might have 

 been confidently predicted. Curi- 

 ously enough, however, traces of 

 both groups have been detected in 

 the fine-grained lithographic slates 

 of Solenhofen and Eichstadt. Of 

 the Medusida, the two living families 

 of the dEqiioridce and Trachyne- 

 mida have been recognised by 

 their impressions ; and an ancient 

 member of the order Rhizostomida 

 represents the Lucernarida in the 

 same formation/ With these ex- 

 ceptions, however, the only living 

 orders of Hydrozoa which have 

 fossil representatives are the Cory- 

 nida and Thecaphora, both of which 

 possess a chitinous or horny in- 

 tegumentary skeleton. In neither 

 case, however, can the evidence be 

 said to be wholly free from sus- 

 picion. 



I. CORYNIDA Or TUBULARIDA 



(Pipe-Corallines.) Animal simple, 

 consisting of a single polypite; or 

 compound, consisting of sei'eral poly- 



pites united to one another by a common flesh or ccenosarc. The 

 cxnosarc generally secretes a hard chitinous outer covering or 

 " polypary ; " but the separate polypites are never protected by cup- 



Fig. ai. Coryniila. 

 f Tubularia indivisa. 



Fragment 

 atural size. 



