RUDIMENTARY STRUCTURES. 243 



become gradually adapted to a completely aquatic 

 condition ; and the existence of these traces of a hind 

 limb is, therefore, of especial importance from this 

 point of view. 



Much the same story is told by the Dugongs and 

 Manatees, all the living species of which have lost every 

 trace of a hind limb ; although in a fossil Miocene 

 species known as the Halithere, there was a pelvis, 

 and a small but well-formed thigh-bone, clearly show- 

 ing relationship with a four-limbed creature. The 

 so-called coracoid process of the shoulder-blade of 

 ordinary mammals is another excellent instance of a 

 rudimentary structure. As we have already mentioned 

 in the chapter on Egg-laying Mammals, this small pro- 

 cess, which in the young animal ossifies from a distinct 

 centre, is the sole remnant of the large coracoid bone 

 found in the shoulder-girdle of birds and reptiles. 



Teeth likewise furnish many very interesting cases 

 of rudimentary development. It has, indeed, been 

 stated that rudiments of teeth occur in very young 

 Parrots, but this does not appear to be really the case ; 

 no trace of these organs being found in any existing 

 birds. Since, however, the birds of the Secondary 

 epoch had useful teeth, we may expect some day to 

 find their rudiments in some of the Tertiary birds. 

 One of the most remarkable instances of rudimental 

 teeth occurs in the Greenland Whale, in which minute 

 teeth are developed beneath the plates of " whalebone " 

 or "baleen," with which the upper jaws are covered, 

 although they never cut the gum. Again, the Nar- 



