| or both dimensions second vertebral | | less than width of scute. | | second vertebral | | scute. --------------------+------------------------+-------------------- Ribs do not tend | Ribs not prominent on | Ribs tend to be to be prominent on | costals. | prominent on costals. | | costals. --------------------+------------------------+-------------------- Hay attached considerable taxonomic importance to the characters of the nuchal and I find its characters to be fairly constant in the specimens of Emydidae examined. Although the nuchal of _Chrysemys limnodytes_ is incomplete, it can be distinguished from the nuchals described by Hay as types of his several fossil Emydids. Differences in the nuchal, together with those in the carapace and plastron, serve to distinguish the species from other genera of the Emydidae. When the specimen is compared with _Chrysemys timida_ Hay, of the Nebraska Pleistocene, many similarities, mostly of generic rank, are seen. _Chrysemys limnodytes_ is broader in relation to length than is either _C. timida_ or any Recent specimen examined of the same size. The greatest allowance possible in estimating the length of _C. limnodytes_ fails to bring the ratio of its breadth to length within the range of Recent specimens of similar size. Data from 96 specimens of Recent _Chrysemys picta_ show that the ratio of length to width is not affected by sex, but that the ratio does vary with the age of the specimen. In young animals the length and width are approximately equal, but with further growth the length becomes relatively greater. Specimens in the length group of 135 to 144 mm. have the widths ranging from 71 to 81 per cent of the lengths. In all specimens larger than this, the ratio is in the low seventies, and the largest specimen, 177 mm. in length, has the width of the carapace amounting to only 74 per cent of the length. The fossil species, _C. timida_, with a length of 160 mm., has the width amounting to 75 per cent of the length, and _C. limnodytes_, with an estimated length of 180 mm., has the width amounting to 80.5 per cent of the length. _C. timida_ is widest anteriorly, whereas _C. limnodytes_ and the other species of the genus are widest posteriorly. Less obvious differences between the two fossils are the narrower anterior margin of the nuchal, the concave anterior end of the carapace, the sculptured surface of the carapace, and the relatively wider neurals and longer vertebrals of _C. limnodytes_. DIMENSIONS OF THE TYPE SPECIMEN (In millimeters) Total length of carapace, 180 (estimated); greatest width of carapace, 145 (estimated); height of carapace, more than 50.