SUB-CLASSES OF BIRDS. 229 
strong rudder-feathers in twos, so that the whole tail 
appears regularly feathered. This same formation of the 
tail part of the vertebral column occurs transiently in the 
embryos of other birds, so that the tail of the Archzeopteryx 
evidently represents the original form of bird-tail inherited 
from reptiles. Large numbers of similar birds with lizard- 
tails probably lived during the middle of the secondary 
period ; accident has as yet, however, only revealed this one 
fossil. 
The Fan-tailed, or Keel-breasted birds (Carinate), which 
form the second sub-class, comprise all living Birds of the 
present day, with the exception of those of the ostrich 
kind, or Ratitz. They probably developed out of Feather- 
tailed Birds during the first half of the secondary period, 
namely, in the Jura or chalk period, by the hinder tail 
vertebree growing together, and by the tail becoming 
shortened. Only very few remains of them are known 
from the secondary period, and these moreover only out of 
the last section of it, namely, from the Chalk. These remains 
belong to a swimming bird of the albatross species, and a 
wading bird like a snipe. All the other fossil remains of 
birds as yet known have been found in the tertiary 
strata. 
The Bushy-tailed, or Ostrich-like Birds (Ratitze), also 
called Running Birds (Cursores), the third and last sub- 
class, is now represented only by a few living species, by 
the African ostrich with two toes, the American and 
Australian ostrich with three toes, by the Indian cassowary 
and the four-toed kiwi, or Apteryx, in New Zealand. 
The extinct giant birds of Madagascar (pyornis) and the 
New Zealand Dinornis, which were much larger than the 
