58. HEART OF ALLIGATOR _Facing_ 202 59. VEINS OF THE POSTERIOR REGION OF ALLIGATOR 204 60. VEINS OF THE ANTERIOR REGION OF THE ALLIGATOR 209 61. ARTERIES OF THE POSTERIOR REGION OF THE ALLIGATOR 213 62. ARTERIES OF THE ANTERIOR REGION OF THE ALLIGATOR 215 VI.-XXVIII. PLATES VI TO XXVIII. A SERIES OF FIGURES TO ILLUSTRATE THE DEVELOPMENT OF THE AMERICAN ALLIGATOR _Following_ 342 THE ALLIGATOR AND ITS ALLIES CHAPTER I THE BIOLOGY OF THE CROCODILIA CLASSIFICATION As in most groups of animals, there is considerable difference of opinion as to the proper classification of the Crocodilia. One of the older textbooks (Claus and Sedgwick) divides the order Crocodilia into three sub-orders: the _Teleosauria_, _Steneosauria_, and _Procœlia_, the last only being represented by living forms. The Procœlia or Crocodilia proper are divided into three families,--the _Crocodilidæ_, the _Alligatoridæ_ (including the caiman as well as the alligator), and the _Gavialidæ_. This division into families seems to be based mainly on the shape of the head, or, at any rate, it throws those forms together that have heads of the same outline. It is this outline of the head that Ditmars (_Reptiles of the World_) uses in classifying the Crocodilia, which, he says, are all included in the single family--_Crocodilidæ_. The following list, taken from his _Reptiles of the World_ (pp. 68-69), will give a clear idea of the number, distribution, and maximum size of the members of the order Crocodilia. More will be said of some of the members of this list later.