8 THE PHENOMENON OP 



that the types of the individual classes were less distinct 

 in the first representatives ; the special character of the 

 class, which is ever more distinctly impressed in the suc- 

 ceeding series of Fishes, not yet being fully developed. 

 Equally remarkable is the relation of the first Mammals 

 of the earth, the celebrated Marsupialia of Stonesfield,* 

 to the class Mammalia generally. The Marsupialia (in- 

 cluding the Monotremata) stand, as is well known, below 

 all Mamraaha, nearest to the oviparous animals of the pre- 

 ceding classes, the Birds and Fishes, and not merely in 

 regard to the structure of the organs of generation, but 

 also in that of the brain ; while, on the other hand, they 

 adjoin in their external form, in the structure of the 

 extremities and the teeth, the higher orders either of the 

 Herbivorous or the Carnivorous series, the Rodentia and 

 the Ferae. We thus see again here the character of the 

 class imperfectly represented in the beginning, and the 

 subsequently diverging series of the class united even 

 to indistinctness. In the geological occurrence of the 

 separate sections of the Invertebrate animals, also, we 

 find the developmental connexion many times confirmed, 

 m that a determinate fundamental type appears first in a 

 few simple forms, then in the course of epochs the 

 number and multiformity of the representatives increase 

 more and more, till finally, when the series of forms is 

 developed to its extreme term, it often suddenly vanishes 

 again from nature, or leaves but a few representatives 

 behind. One of the most remarkable examples of this 

 kind is presented by the family of the Ammonites. f 



* Phascohtherium Buclclandi, and Thylacotlierium Prevostii, Owen. Vide 

 Buokland, 'Mineralogy and Geology,' i, 73. 



f The transition period possesses the single genus of the family of 

 Ammonites, the Ooniatites, the trias period the genus Ceratites, the Jurassic 

 age 11 sections of the genus Ammonites, the chalk period 14 sections of the 

 same genus, 10 of which are pecuUar, and besides these the genera Crioceras, 

 Aticijloceras, Scaphites, Samites, Ptychoceras, Bacidites, Turrilites, and 

 Helicoceras, all of which occur exclusively in the chalk formation, with tlie 

 exception otAncyloceras, of which genus the Jura formation already possessed 

 species. Then the family of the Ammonites vanishes totally. See Leopold 

 von Buoh on Ammonites, imd D'Orbigny, ' Palseontologie Franijaise,' i, 433. 



