. V. ARRANGEMENT. 191 



In the Genus MAMMODOLITHUS The united 

 bones of the head, including- the teeth. 



ORNITHOLITHUS The boneg of 



the head united to the bill. 



AMPHIBIOLITHUS. The united 



dorsal vertebrae. 



ICHTHYOLITHUS The Jlns and 



head, united with the skeleton or with the external 

 covering of the body. 



ENTOMOLITHUS The upper ex- 

 ternal covering of the thorax, united to that of the 

 abdomen. 



rested the classification of the recent species these are the parts 

 of the skeleton, therefore, best known to the modern zoologist ; 

 nnd from these, of course, he will most readily determine the na- 

 tural order or genus of the fossil subjects, and draw with facility 

 permanent specific distinctions The same reasons will apply to 

 the preference given to the head and bil/ > over oilier parts of the 

 skeleton, in Ornitholithus In amphibwus animals , the dorsal 

 vertebra afford not only certain, but well known distinctive marks 

 of the genera, to which the fossil remains, hitherto discovered, of 

 this class belong For instance, in Testudo the vertebra are united 

 with the ribs into one solid piece, which constitutes the back shell 

 or covering of the animal. This part, therefore, sufficiently dis- 

 tinguishes the genus from Rana, in which the vertebra are desti- 

 tute of ribs, and both from Lactrta t in which proper and distinct 

 ribs are attached to the dorsal vertebra These are the only 

 genera of amphibia (the Linnean order of Nantes being excluded 

 the class) that have been observed giving form to extraneous fos- 

 sils; and of these, the few examples that have occurred, have 

 mostly exhibited the parts in question The propriety therefore 



