Turtles and tortoises are among the most often seen and easily recognized non-avian reptiles.  All species are encased in a pair of bony shells, a dorsal carapace and a ventral plastron. The shells (which are covered with a horny layer of keratin) are formed from sections of dermal bones that are fused to each other and to the ribs and thoracic vertebrae. Although all turtles (such as the painted turtle shown in the image above) are oviparous, burying their eggs in the soil, in some species, the temperature at which the eggs are incubated determines the sex of the offspring (cooler temperatures produce males).