| RECENT Chrysemys | Chrysemys limnodytes | RECENT Pseudemys |
|---|---|---|
| Not serrated. | Posterolateral marginals not serrated. | Serrated. |
| Not notched. | Carapace with broad shallow notch at anterior end. | Notched. |
| Occasional faint notch at femoro-anal suture. | Plastron does not havea pronounced notch at femoro-anal suture. | Distinct notch at femoro-anal suture. |
| Posterior lobe of plastron flares laterally. | Posterior lobe of plastron does not flare laterally. | Posterior lobe of plastron does not flare laterally. |
| Carapace smooth. | Carapace has smooth contours. | Carapace has depressions and elevations. |
| Old specimens occasionally have five or six ridges near border of costals. | Seven or eight ridges on lower half of costals. Remaining surface of costals and neurals rugose. | Ridges cover costals. |
| No ridges on marginals. | No ridges on marginals. | May or may not have ridges on marginals. |
| Nuchal smooth. | Nuchal smooth. | Nuchal has ridges. |
| Carapace not greatly arched. | Probably arched less than in Pseudemys, but more than in any Recent Chrysemys. | Greatly arched. |
| Keel often present at birth, but soon lost. | No keel present. | Keel often present. |
| Anterior and posterior widths of first vertebral scute approximately same as width of second vertebral scute. | Anterior and posterior widths of first vertebral scute less than width of second vertebral scute. | Anterior width of first vertebral scute less than posterior width, or both dimensions less than width of second vertebral scute. |
| Ribs do not tend to be prominent on costals. | Ribs not prominent on costals. | Ribs tend to be prominent on 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.
Length of plastron, 165 (estimated); width of plastron, 130 (estimated); length of anterior lobe, 45 (estimated); width of anterior lobe, 75 (estimated); length of posterior lobe, 62; width of posterior lobe, 82; length of bridge from axillary to inguinal notch, 60.