Description
All box turtles have a highly domed carapace. All species are domed, with a first central scute at an angle of more than 50° while the next central scutes are often flattened. While T. carolina species have a prominent medial keel (a ridge over the length of the carapace), this feature is (nearly) absent in the other species. As a result of the hinge in their plastron (between the abdominal and pectoral scales), box turtles can close very tightly to protect themselves from predators. The plastral formula (size relation between the scales) varies between species, but the order of the largest three scutes is anal > abdominal > gular in all species. The ability to close develops when it is one quarter grown and is generally only lost due to corpulence. Based on morphological characteristics, two distinct lineages can be distinguished: the ornata/nelsoni lineage as well as the carolina/coahuila cluster. The ornata/nelsoni cluster is the smallests (maximum carapace lengths of 14–15 cm), while the other cluster is larger (with T.c.major with a carapace length of 20 cm). The radiation pattern varies strongly: from the none-patterned C. coahuila, to the characteristic stripes in C. ornata and the yellow/brown spots in C. nelsoni. C. carolina is the most varied genus with spots, bars and lines which pattern often also varies from scute to scute.
To distinguish between a male and a female box turtle, it is simplest to turn the turtle over and examine its plastron. The male usually has a concave area centered posterior to the hinge. This adaptation apparently aids the male's ability to climb on top of the female during mating. The one pictured above left is a male.
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