_Comparisons._--_T. s. hartwegi_ can be distinguished from all other subspecies of _T. spinifer_ by the presence of small dots and ocelli on the carapace that are all of approximately the same size in combination with only one dark marginal line. _T. s. hartwegi_ resembles _asper_ in having small blackish ocelli or dots on the carapace but differs from _asper_ in having only one dark marginal line. _T. s. hartwegi_ differs from _spinifer_ only in the small size of the ocelli. _T. s. hartwegi_ resembles _spinifer_ and _asper_, but differs from _pallidus_, _guadalupensis_ and _emoryi_ in having blackish spots and ocelli on the carapace and lacking small whitish spots. _T. s. hartwegi_ resembles _spinifer_, _asper_ and _pallidus_ but differs from _guadalupensis_ and _emoryi_ in having conical or knoblike tubercles on the anterior edge of the carapace on large females. _T. s. hartwegi_ differs from the subspecies _asper_, _guadalupensis_ and _emoryi_ in having a narrower head, and from _emoryi_ in having a wider carapace. _T. s. hartwegi_ resembles _spinifer_ and _asper_ but differs from the other subspecies in having the carapace widest at a plane approximately one-half way back on the carapace. _T. s. hartwegi_ and _spinifer_ have longer snouts than do _pallidus_ and _guadalupensis_ or _emoryi_. _T. s. hartwegi_ differs from _asper_ but resembles the other subspecies in having a relatively longer plastron. _Remarks._--The validity of _T. s. hartwegi_ has never been questioned. It intergrades with _spinifer_ over a broad area paralleling the Mississippi River. For convenience, specimens occurring west of the Mississippi River are referred to the subspecies _hartwegi_. Figure 8 shows much variation in size of ocelli on different individuals from the same state. For example, UMMZ 92667, plastral length 6.7 centimeters has a uniform pale brown carapace lacking any dark marks, whereas UMMZ 92652, plastral length 5.9 centimeters has some ocelli three millimeters in diameter on the carapace. Both are from Iowa. One specimen from Kansas, KU 1954 (Doniphan County, plastral length 11.8 cm.), has ocelli four millimeters in diameter, and USNM 7648 captured farther west at Fort Laramie, Wyoming, an adult male having a plastral length of 11.0 centimeters, has some ocelli five millimeters in diameter on the carapace. TTC 1090, an adult male from the panhandle of Texas has some ocelli so much as 5.5 millimeters in diameter. The size of the ocelli seemingly varies in the same local population.