INTEODUCTION. IX. 



are characterized by a distinct set of organic forms, and in 

 many instances, the subordinate beds have peculiar species. 

 The Tertiary period contains many species of Testacea, the 

 forms of which are analogous, and sometimes identical 

 with, existing types ; as weU as the remains of the large 

 Mammalia and Fishes belonging to the Cycloid and Ctenoid 

 orders — orders which comprise nearly all the known living 

 species, whereas the fossil fishes that are found in the 

 Secondary and Palaeozoic periods, belong to the Ganoid 

 and Placoid orders, of which there are but very few exist- 

 ing forms. The Secondary period is characterized by two 

 genera, which are peculiar to it. Ammonites and BeUmnites, 

 belonging to the Cephalopoda. The Saurian family also 

 abound at this period. 



The Palaeozoic period is marked by other forms of Cepha- 

 lopoda, Goniatites, OrtJioceras, Cyrtoceraa, &c.,as also by the 

 great abimdance of the Brachiopoda, as Prodtictm,Lept^ia, 

 Spirifer, Orthis, &c., and some singular forms of Crustacea, 

 the Trilobites ; as weU as many species of Crinoidea. 



Limited as is the extent of the British Islands, yet they 

 contain (with one exception, the Muschelkalk) a fuU deve- 

 lopment of the Geologic series, and thus afford an index 

 by which the formations in distant parts of the globe may 

 be compared or studied. 



By consulting a geological map of England, the Student 

 wiU readily perceive that the physical features of the 

 country are chiefly dependent on the geological structure, 

 and that the formations succeed each other in chronological 

 order as we proceed from the western to the eastern coast, 

 thus, the oldest slate system and the associated unstratified 



