Phyllis Wall, age 12, of Trumbull, Conn., for her question;

How can you tell an alligator from a crocodile?

This is a very popular question, and no wonder. You may have seen movies or pictures of these toothy fellows. You may have handled shoes, belts, wallets or bags made from their leathery hides. But your first sight of one of the living animals is sure to be a shock. They are so different from other animals that we wonder whether they are not in the modern world by mistake. Actually they are survivors from the Age of the Dinosaurs. The big dinosaurs perished some 60 million years ago. Their cousins, the crocodilians, are still with us.

There are three varieties of crocodilians. There are the gavials of India and Malay, the alligators of the New World and the true crocodile of tropical Africa and. Asia. The gavial is a slender animal, with a long pointed nose. Since he lives in far away places we are not likely to confuse him with the alligator and the crocodile,

The grown crocodile is far larger than the largest alligator. He may be more than 30 feet long and weigh a ton. This makes him the largest of all the living reptiles. The largest alligator rarely exceeds 16 feet from nose to tail. Size, however, is not a reliable guide for you might confuse a half grown crocodile with a full grown alligator. The kind of water in which you find the animal gives a more reliable clue. Crocodiles enjoy salt and brackish water. Alligators prefer fresh water.

The shape of the head gives a still more reliable clue. The head of the alligator is wider and more wedge shaped than that of cousin crocodile. The crocodiles snout is longer and less tapered. However, if the animal you happen to be observing opens his mouth, do not stop to take measurements. Those toothy jaws are ready to snap shut like a bear trap.

The one sure way to tell the alligator from the crocodile happens to be from those terrible teeth. Both cousins have a few extra long teeth in their lower jaws. The alligator has pits in his upper jaw into which these dagger teeth fit when his mouth is closed. The crocodile has notches along the side of his upper jaw into which his dagger teeth fit when he closes his mouth.

The variation makes a vast difference in the facial expression. All crocodilians are without lips which gives them a kind of smile when the mouth is closed. When the mouth of the alligator is closed, with the dagger teeth fitted into pits, no teeth are visible. He wears a gentle smile which certainly belies his bloodthirsty nature.

The crocodile can never hide his dagger teeth. They show even when his mouth is closed. This gives him a ferocious, cunning smile, which is true to his nature. These different facial expressions are the best clues to whether an animal is an alligator or a crocodile.