CRETACEOUS. 417 



belonging to extinct families (Squaloraja, Myriacanthus), others 

 referable to the existing Chimaeridae (Ischyodus). The Crosso- 

 pterygians are represented by numerous Ccelacanthidae ( Undina), 

 and the Isospondyli are just beginning to assume some im- 

 portance (Pholidophorus, Leptolepis). Marine reptiles are also 

 extremely abundant everywhere and belong to the three orders, 

 Sauropterygia, Ichthyopterygia, and Crocodilia. 



On land this was the age of reptiles, at least in the 

 northern hemisphere. Both in Europe and North America 

 the Stegocephalia and Anomodontia disappear at least with 

 the Triassic; but the Dinosauria assume an astonishing de- 

 velopment everywhere in the Jurassic, and some of them attain 

 larger dimensions than any land-animal of later date. Towards 

 the close of the period some of the crocodiles also acquire a fluvia- 

 tile and terrestrial habit (Goniopholis, Bernissartia). Flying 

 reptiles (Ornithosauria) are well represented throughout all 

 Jurassic formations ; and birds are first known to appear in the 

 Upper Jurassic with the rare genus Archceopteryx. 



No undoubted trace of the Jurassic land-fauna has hitherto 

 been discovered in the southern hemisphere, for Steneosaurus, 

 found in Madagascar, is rather to be regarded as a marine type; 

 but it is extremely probable that on some continent in that 

 part of the globe the Anomodontia were gradually being trans- 

 formed into Mammalia. At least, in the Jurassic formations 

 both of Europe and North America there are occasional remains 

 of small mammals as large as rats ; and the most plausible 

 explanation of these is, that they were accidental escapes from 

 some other region with a more advanced fauna, just as are the 

 rats and mice of the present day in the comparatively antique 

 realm of Australia. 



CRETACEOUS. 



The fish-fauna in the Lower Cretaceous deposits is closely 

 similar to that of the Jurassic ; but that of the Chalk and 

 other Upper Cretaceous formations, is quite modern in aspect, 

 with only a slight admixture of the Crossopterygian Coelacan- 

 thidse (Macropoma) and the Actinopterygian Protospondyli 

 (Neorhombolepis, Protosphyrcena, and Pycnodontidas). Essen- 

 w. 27 



