ADAPTATIONS TO SPECIAL ENDS 135 



ward prolongation of the hard palate. In most reptiles only 

 the premaxillae develop palatal plates to form this secondary 

 flooring, but in modern crocodiles such plates are also developed 

 by the maxillary, palatine, and pterygoid bones, in consequence 

 of which the choanae open immediately in front of the occipital 

 condyle. As the result of this arrangement the posterior 

 apertures of the nasal passages are brought (by means of an 

 elongation of the latter) into direct communication with the 

 wind-pipe, or trachea, and crocodiles are thus enabled to 

 open their mouths and hold their prey under water and yet, 

 by keeping their nostrils above the surface, to breathe all 

 the time without difficulty. A similar structural adaptation 

 occurs in whales and dolphins; but is wanting in the com- 

 pletely marine ichthyosaurs and plesiosaurs, and it is conse- 

 quently difficult to understand how those reptiles managed to 

 hold and swallow their prey in the water, unless indeed the 

 nasal passage was prolonged backwards by means of some 

 special arrangement of the soft parts. The same remark applies 

 to the Mesozoic pelagic crocodiles (Geosaurus, etc.) and the sea- 

 serpents (Pythonomorpha) ; but the ordinary Mesozoic croco- 

 diles, such as Teleosaurus and Steneosaurus, in which the aper- 

 tures of the posterior nostrils were situated far forward on the 

 palate, may have carried their prey to land. An extinct croco- 

 dile {Leidyosuchus) from the Cretaceous of North America 

 presents an intermediate condition ; the choanae opening in 

 the middle of the pterygoid bones. 



Another group of reptiles in which the posterior apertures 

 of the internal nostrils are carried far back (although not to 

 the same degree as in modern crocodiles) by the development 

 of a secondary floor to the palate is the genus Lytoloma, which 

 includes marine turtles of Lower Eocene and Upper Cretaceous 

 age. In this case, which has been evolved independently of 

 the crocodiles, it has been suggested that the arrangement is 

 connected with the great backward extension of the bony union, 

 or symphysis, of the two branches of the lower jaw : and it 

 has been thought that the latter feature indicates that the food 

 of these turtles consisted of shell-bearing molluscs. 



Be this as it may, it is clear that the extinct turtles of the 

 genus Lytoloma resembled their modern marine cousins and 

 the other members of the order Chelonia generally in that the 



