p. bellii), which grows up to long. Its top shell has a mesh-like pattern of light lines, and the top stripe present in other subspecies is missing or faint. Its bottom shell has a large colored splotch that spreads to the edges (further than the midland) and often has red hues. Until the 1930s many of the subspecies of the painted turtle were labeled by biologists as full species within Chrysemys, but this varied by the researcher. The painted turtles in the border region between the western and midland subspecies were sometimes considered a full species, treleasei. In 1931, Bishop and Schmidt defined the current "four in one" taxonomy of species and subspecies. Based on comparative measurements of turtles from throughout the range, they subordinated species to subspecies and eliminated treleasei. Since at least 1958, the subspecies were thought to have evolved in response to geographic isolation during the last ice age, 100,000 to 11,000 years ago. At that time painted turtles were divided into three different populations: eastern painted turtles along the southeastern Atlantic coast; southern painted turtles around the southern Mississippi River; and western painted turtles in the southwestern United States. The populations were not completely isolated for sufficiently long, hence wholly different species never evolved. When the glaciers retreated, about 11,000 years ago, all three subspecies moved north. The western and southern subspecies met in Missouri and hybridized to produce the midland painted turtle, which then moved east and north through the Ohio and Tennessee river basins. Biologists have long debated the genera of closely related subfamily-mates Chrysemys, Pseudemys (cooters), and Trachemys (sliders). After 1952, some combined Pseudemys and Chrysemys because of similar appearance. In 1964, based on measurements of the skull and feet, Samuel B. McDowell proposed all three genera be merged into one. However, further measurements, in 1967, contradicted this taxonomic arrangement. Also in 1967, J. Alan Holman, a paleontologist and herpetologist, pointed out that, although the three turtles were often found together in nature and had similar mating patterns, they did not crossbreed. In the 1980s, studies of turtles' cell structures, biochemistries, and parasites further indicated that Chrysemys, Pseudemys, and Trachemys should remain in separate genera. In 2003, Starkey et al. proposed that Chrysemys dorsalis, formerly considered a subspecies of C. picta, to be a distinct species sister to all subspecies in C. picta. Although this proposal was largely unrecognized at the time due