_Holotype._--University of Kansas Museum of Natural History No.
7676, vertebrate paleontological collection, a turtle consisting of
a fragmental anterior portion of a carapace, left part of the
plastron, and several marginals collected by the 1946
paleontological field party of the University of Kansas Museum of
Natural History.
_Geological Age and locality._--Marl beds of the Laverne formation,
early Pliocene age, in SW 1/4 Sec. 15, T. 4 N., R. 25 ECM, Beaver
County, Oklahoma. The specimen was removed from the marl
immediately below the fossil leaf zone (see Chaney and Elias, 1936;
Frye and Hibbard, 1940).
_Diagnosis._--Size large (see measurements) and differing from
other species of _Chrysemys_ in having: The anterior end of the
carapace broadly concave, the posterolateral marginals not greatly
flared, the posterior end of the plastron broadly indented, the
carapace more sculptured and relatively wider.
_Description of type._--The specimen had been badly damaged before
preservation, and had suffered further damage from exposure before
discovery. The anterior and posterior lobes of the plastron had been
folded over the bridge, forming a three-ply thickness of bone. Of the
carapace, only the following parts are known: Fragment of the nuchal;
right 1st, 7th, 8th, and 9th marginals, left 1st, 2d, 7th, 8th, 9th, and
11th marginals; costals 1-5 on the right side; costals 1-4 on the left
side; and 1st, 2d, 3d, and 4th neurals. The left half of the plastron is
relatively complete, lacking only the epiplastron and entoplastron. The
left 7th, 8th, and 9th marginals are joined to the plastron at the
inguinal buttress, and the right 7th, 8th, and 9th marginals are
attached to the fifth costal. The carapace has smooth contours with no
keel present, but on the lower half of the costals there are seven or
eight ridges, and the remaining surface of the costals and neurals are
rugose. The marginals lack ridges, and the posterolateral marginals are
not serrated. The anterior end of the carapace has a broad shallow
notch. The first neural is rounded, and the 2d, 3d, and 4th are
hexagonal, with the broad ends forward. Anterior margins of the 2d and
3d neurals are concave, and the anterior margin of the 4th neural is
straight.
The sulci bounding the scutes are moderately impressed. The width of the
first vertebral scute, anteriorly and posteriorly, is less than the
width of the second vertebral scute. The costal scutes join the marginal
scutes on the marginal plates.
The plastron, broadly indented at the posterior end, does not have the
posterior lobe flared laterally as it is in Recent species of
_Chrysemys_, and lacks any pronounced notch at the femoro-anal suture.
The humero-pectoral sulcus crosses the plastron behind the entoplastron
in a straight line, and reaches the border anterior to the axillary
notch. The pectoro-abdominal suture is anteriorly convex at the sides
and concave at the midline.
A comparison of this carapace and plastron with a series of specimens of
Recent _Chrysemys picta_ and _Pseudemys scripta_ of approximately the
same size reveals characters indicated in the following chart:
==================================================================
RECENT _Chrysemys_ | _Chrysemys limnodytes_ | RECENT _Pseudemys_
--------------------+------------------------+--------------------
Not serrated. | Posterolateral | Serrated.
| marginals not serrated.|
--------------------+------------------------+--------------------
Not notched. | Carapace with broad | Notched.
| shallow notch at |
| anterior end. |
--------------------+------------------------+--------------------
Occasional faint | Plastron does not have | Distinct notch at
notch at femoro-anal| a pronounced notch at | femoro-anal suture.
suture. | femoro-anal suture. |
--------------------+------------------------+--------------------
Posterior lobe of | Posterior lobe of | Posterior lobe of
plastron flares | plastron does not | plastron does not
laterally. | flare laterally. | flare laterally.
--------------------+------------------------+--------------------
Carapace smooth. | Carapace has smooth | Carapace has
| contours. | depressions and
| | elevations.
--------------------+------------------------+--------------------
Old specimens | Seven or eight ridges | Ridges cover
occasionally have | on lower half of | costals.
five or six ridges | costals. |
near border of | Remaining surface of |
costals. | costals and neurals |
| rugose. |
--------------------+------------------------+--------------------
No ridges on | No ridges on | May or may not
marginals. | marginals. | have ridges on
| | marginals.
--------------------+------------------------+--------------------
Nuchal smooth. | Nuchal smooth. | Nuchal has ridges.
--------------------+------------------------+--------------------
Carapace not | Probably arched less | Greatly arched.
greatly arched. | than in _Pseudemys_, |
| but more than in any |
| Recent _Chrysemys_. |
--------------------+------------------------+--------------------
Keel often present | No keel present. | Keel often present.
at birth, but soon | |
lost. | |
--------------------+------------------------+--------------------
Anterior and | Anterior and posterior | Anterior width of
posterior widths of | widths of first | first vertebral
first vertebral | vertebral scute less | scute less than
scute approximately | than width of second | posterior width,
same as width of | vertebral scute. | 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_.