GENERAL CHARACTERS 3 



Finally, there must have been mammal-like reptiles which ac- 

 quired two occipital condyles by the suppression of the median 

 element of the reptilian one, and at some stage of their progres- 

 sive evolution exchanged their scales (if they possessed them) 

 for hair, and likewise raised the temperature of their blood 

 markedly above that of the surrounding medium. 



Obviously, then, as in all analogous instances, it is not of 

 much use attempting to define distinctions which may have 

 never existed between extinct reptiles and the other three 

 classes of terrestrial vertebrates. It must therefore suffice to 

 give the foregoing definition of existing reptiles, and to consider 

 that such extinct vertebrates as come within its limits, or which 

 approximate more or less closely thereto, are likewise to be in- 

 cluded within the limits of the class. 



Existing reptiles include only a small number of leading 

 groups, or orders ; but such groups do not altogether coincide 

 with the popular classification of these creatures. In popular 

 language reptiles are roughly classed as crocodiles (inclusive of 

 alligators and gharials), tortoises and turtles, lizards, and 

 snakes. Crocodiles, in this wider sense, constitute an ordinal 

 group (Crocodilia) by themselves, as do tortoises and turtles a 

 second (Chelonia). Here, however, the agreement between 

 popular and scientific classification ceases, for whereas in the 

 former the lizard-like tuatera (Sphenodon) of New Zealand is 

 reckoned as a lizard, in the latter it is regarded as the sole 

 surviving representative of an extremely generalised order 

 (Rhynchocephalia), with but little in common with the group 

 in which the true lizards are included. Again, lizards and 

 snakes are popularly classed as widely sundered groups, 

 whereas in the scientific scheme the two (exclusive of the 

 aforesaid tuatera) are brigaded in a single group (Squamata) 

 ranking in value with the Crocodilia, Chelonia, and Rhyn- 

 chocephalia. Nor is this all, for chamaeleons, which are 

 commonly regarded as lizards, are considered by some natural- 

 ists to form a third subgroup (Rhiptoglossa) of the Squamata, 

 with the same value as the two respectively containing the 

 lizards (Lacertilia) and the snakes (Ophidia). Recent reptiles 

 and their immediate extinct kindred are consequently classed 

 as follows : — . 



