1. Plastral pattern consisting
pattern, all parts of which of separate brown smudges (at
are interconnected. Plastral least anteriorly). Plastral
pattern partly obliterated by pattern obliterated by
melanism in old individuals melanism only in adult males.
of both sexes.

2. Markings of carapace in form 2. Markings of carapace having
of indistinct ocelli. linear and vertical.

3. Cutting edge of mandible 3. Cutting edge of mandible
serrate. smooth.

4. Foreclaws of mature males 4. Foreclaws of mature males
unmodified. greatly elongated.

5. Gular shorter than pectoral 5. Gular longer than pectoral
(91 per cent of specimens), (90 per cent of specimens)
gular and femoral subequal. and longer than femoral (all
specimens).

6. Shell relatively higher, 6. Shell relatively lower,
posterior lobe of plastron posterior lobe of plastron
relatively narrower (Fig. 3). relatively wider (Fig. 3).

7. Lateral edges of posterior 7. Lateral edges of posterior
plastral lobe reflected plastral lobe unmodified.
downward.

Four specimens of _P. s. ornata_ (MCZ 46392-3, 46397, 46400, two adult
females and two adult males) from the Rio Soto la Marina drainage of
Tamaulipas differ from _P. s. taylori_ as follows: plastral pattern
diffuse and brownish, not black; gular longer than pectoral; cutting
edge of lower jaw only slightly serrate; stripe on mandibular
symphysis isolated, not joined with ventral neck stripes to form
inverted Y; postorbital stripe (yellow in preservative) connected to
eye by narrow isthmus and continuous with neck stripe to shoulder.

In _P. s. taylori_ there is an obtuse ridge or prominence across the
bridge, on a line joining the free lateral edges of the plastron; the
area between the ridges is nearly flat. The bridge forms a distinct
plane on each side between the mentioned ridge and the outer edges of
the marginals. In cross section this plane forms an angle of 30 to 45
degrees with the horizontal plane of the plastron. The higher bridge
and deeper shell of _taylori_ result in a slightly higher center of
gravity in this subspecies than in the specimens of _elegans_ and
_ornata_ I have examined. In the two subspecies last named the