The stomach, moreover, in the crocodiles is fashioned somewhat after that of the birds, with an imperfect division into crop and gizzard. Some crocodiles of today have the habit of swallowing hard pebbles, as do many birds. There is an old myth that the crocodile of the Nile swallows a pebble on each of its birthdays, thus giving reliable information as to its age by the number found in its gizzard at its death! And this habit has been suggested for some of the most ancient crocodiles, the teleosaurs, by the recurring presence of siliceous pebbles found with the remains of their skeletons. And we have seen this pebble-swallowing habit was also characteristic of the plesiosaurs, with whose remains “stomach-stones,” or gastroliths, as they have been called, are often found. [Illustration: FIG. 104.—Pelvis of crocodile: _il_, ilium; _is_, ischium; _pu_, pubis.] All of these various characters of the skeleton and fleshy parts are pretty conclusive evidence that the crocodiles, ugly creatures that they are, today enjoy the highest rank among cold-blooded animals. They are perhaps in some respects of not so high a type of reptiles as were some of the extinct reptiles, but that they have survived so long, so many millions of years, is pretty good evidence of endurance, to say the least. Living crocodiles belong to three distinct groups or families: the true crocodiles and alligators; the long-snouted crocodiles or Borneo gavials; and the true gavials of India. Members of the first of these families are really only subaquatic, or amphibious in habit; they move about on land with entire freedom, and often seek their food there. Certain marked aquatic characters they do possess, in the skull and tail, as we shall see. They are indigenous to southern China, India, Africa, Madagascar, the southern part of the United States, Central America, and the northern part of South America. The members of this family are distinguished by the more or less broad and flat head, the possession of comparatively few teeth of large size, and by having the toes less completely webbed. The crocodiles proper differ from the caimans and alligators especially in the arrangement of the teeth. During later geological times, that is, during early Tertiary times, the geographical range of the Crocodilidae was much more extended than it is at present, the remains of many often very large species, being found in the lake deposits of the northwestern parts of the United States, Great Britain, Germany, France, etc. Yet earlier, in the late Cretaceous rocks of the United States, in Texas, and Wyoming especially, there have been found rather scanty remains of a gigantic crocodile which must have been nearly fifty feet in length when living.