Some 50 bones, including the turtle's spinal column and ribs, form the upper shell, called the carapace. Another 11 bones comprise the lower section, or plastron. They are joined on each side by bridges of shell or by ligaments. The shells of most turtles have the added protection of a layer of horny shields that overlap the seams of the bony plates. In softshell turtles, the covering is tough skin that is dark olive or brown on the carapace but so light colored on the plastron that the underlying bone structure is usually visible.

The largest living turtle in North America, other than the sea turtles that nest on ocean beaches, is the alligator snapping turtle. A specimen in Chicago's Brookfield Zoo weighed 249 pounds when it was measured a few years ago, and a bigger one could be lurking on the bottom of one of the rivers draining into the Mississippi from Illinois to the Gulf of Mexico. This is truly a creature of primordial visage with its immense head, gaping jaws and 30-inch carapace adorned with three craggy keels, all of which indeed give it a gatorlike appearance.

Adult alligator snappers, no surprise, have only one mortal foe--in the form of humans who hunt them relentlessly for the turtle-meat market. But the lives of most native turtles are full of peril from hatching to old age. Predators of the ubiquitous painted turtle, for example, include muskrats, raccoons, alligators, snapping turtles, snakes, bullfrogs, large fish, herons and ospreys. And among the smaller, slow-moving turtles, a passive defense usually is the best defense against terrestrial predators. Some species, notably map turtles, will try to flee if given a chance, but most turtles pull their head, legs and tail into the shells and await further developments. Box turtles and Blanding's turtles can even slam the door behind them with a hinged plastron that locks against the carapace.

But the safety of the shells is not absolute. Researchers in New Jersey recently examined 53 living wood turtles that had been injured by predators, and 27 of them were missing limbs while 11 others had mutilated carapaces. (Turtles sometimes live for years with well-healed major injuries.) Wood turtles, by the way, are overland speed demons compared to other species. One adult wood turtle covered a distance of 450 feet in 25 minutes, a rate of 0.2 miles per hour. A migrating bog turtle, on the other hand, traveled just 56 feet in a day and took two weeks to cross a meadow 600 feet wide.

Turtle temperament when the creatures are handled by humans varies from species to species and even among individuals. Some, like the spotted turtle, are predictably mild mannered while others will try to bite or scratch and often urinate.

The stinkpot or common musk turtle, a largely nocturnal species found across much of the eastern United States, discharges a vile-smelling fluid that makes this highly aquatic species unpopular with sport fishermen when it grabs a baited hook and has to be reeled in.

The turtle with the most pugnacious, don't-mess-with-me attitude is the common snapping turtle, which can weigh 60 pounds or more and prefers a good offense over defense. This is due in large part to the species' unusually active lifestyle: Snappers sometimes travel long distances by turtle standards on powerful legs that are too large to be contained by a small, cross-shaped plastron. (One young snapper was found atop Big Black Mountain in Kentucky in 1950, five miles from the nearest pond.) Facing trouble head on, a snapper will tilt its carapace forward like a knight's shield to protect its vulnerable parts and lunge with astonishing speed--often locking its jaws on an aggressor's extremities. That hurts!