In areas where _T. ornata_ and _T. carolina_ are sympatric (for example, in Illinois, Kansas, and Missouri) the two species occupy different habitats, _ornata_ preferring open grassland and _carolina_ wooded situations. Under natural conditions, the average incubation periods of these two species can be expected to differ, _T. carolina_ having a somewhat longer period due to lower temperatures in nests that are shaded. In the light of these speculations, the remark of Cahn (1937:102)--that _T. ornata_ nested later in the season (in Illinois) and compensated for this by having a shorter incubation period--is understandable. The range of temperatures tolerated by developing eggs probably varies with the stage of embryonic development. When temperatures in the laboratory were 102 to 107 degrees Fahrenheit for approximately eight hours, due to a defect in a thermostat, the young in two eggs of _T. ornata_, that had begun to hatch on the previous day, were killed, as were the nearly full-term embryos in a number of eggs of _T. carolina_ (southern Mississippi) kept in the same container. A five-day-old hatchling of _T. ornata_, kept in the same container, survived the high temperatures with no apparent ill effects. Cagle (1950:41) found that eggs of _Pseudemys scripta_ could not withstand temperatures of 10 degrees for two weeks nor would they survive if incubated at 40 degrees. Cunningham (1939) reported that eggs of _Malaclemys terrapin_ could not survive prolonged exposure to temperatures of 35 to 40.6 degrees but tolerated temporary exposure to temperatures as high as 46 degrees. In the summer of 1955, a clutch of three eggs, all of which contained nearly full-term embryos, was placed in a refrigerator for 48 hours. The temperature in the refrigerator was maintained at approximately 4.5 degrees; maximum and minimum temperatures for the 48 hour period were 2.8 and 9.5 degrees, respectively. When the eggs were removed from the refrigerator they showed gains in weight and increases in size comparable to eggs, containing embryos of the same age, used as controls. The experimental eggs began to hatch two days after they were removed to normal temperatures--approximately 24 hours later than the controls. In the late stages of incubation, the outer layer of the shell becomes brittle and is covered with a mosaic of fine cracks or is raised into small welts. Several days before hatching, movements of the embryo disturb the surface of the shell and cause the outer layer to crumble away, especially where the head and forequarters of the embryo lie against the shell. Some embryos could be seen spasmodically thrusting the head and neck dorsally against the shell.