VERTEBRATA 13 



ribs intermyotomic ; vertebral column consist- 

 ing chiefly of centra; an amnion and al- 

 lantois ; Amniota. 



Limbs as in Batrachia; one occipital condyle; 

 a suspensorium of the lower jaw ; 

 mandible segmented ; a coracoid bone ; 

 ankles between first and second rows of 

 carpal and tarsal bones; heart with three 

 or four chambers ; Monocondylia. 



Limbs as in Batrachia; two occipital condyles; 

 no suspensorium of the lower jaw ; man- 

 dible not segmented ; coracoid generally 

 not distinct ; ankles between propodial 

 bones and carpus or tarsus ; heait with 

 four chambers ; Mammalia. 



The period of appearance and duration of each of these 

 classes is exhibited in the accompanjdng table. It will be 

 observed that the order of appearance corresponds with the 

 natural order of structural complexity, the simplest appear- 

 ing earliest and the most complex last. It must be 

 mentioned that it is probable that each of the classes ap- 

 peared a little earlier than the time assigned them in this 

 table. The discovery of the fossil remains is regarded as 

 the first positive indication of the presence of a species of a 

 given class. Spines uncertainly referred to sharks (Pisces) 

 have been found in the Middle and Lower Siluric in 

 Europe and North America respectively. Foot impressions, 

 probably of Batrachia, have been found in the Carbonic at 

 lower horizons than the bones. Tracks, probably of birds, 

 have been found in the Trias. A fore limb, probably of a 

 mammal, has been found in the Permo-Triassic Karoo 

 formation of South Africa. 



The geological range of these classes is as follows: 



