Comparison of the relative lengths of snout (HW/SL, Fig. 10) in different populations of _T. spinifer_ shows a character gradient. To facilitate a comparison utilizing large samples, the subspecies _spinifer_ was combined with _hartwegi_, and _pallidus_ with _guadalupensis_. The snout is longer in the subspecies _spinifer_ and _hartwegi_ than in _emoryi_; the length of the snout of _emoryi_ resembles that of _T. ferox_. The snout is proportionately the longest in _T. muticus_. The average ratio of HW/SL for 12 individuals of _T. ater_ is 1.37, and is nearer that of _pallidus_, _guadalupensis_, _emoryi_ and _ferox_ than that of _muticus_ or the other subspecies of _T. spinifer_. [Illustration: FIG. 10. Ratio of width of head to length of snout (HW/SL) in some American species and subspecies of the genus _Trionyx_. Symbols as in Fig. 3. Values for _spinifer_ are combined with those of _hartwegi_, and those of _pallidus_ with _guadalupensis_. The snout is proportionately the longest in _muticus_.] Size of the ocelli increases from west to east in populations of _T. spinifer_ in the upper Mississippi River and Great Lakes drainages. The ratio of OD/PL (Fig. 11) varies considerably but gradually increases from Kansas northeastward to Michigan. The minimal diameter of any ocellus recorded was one millimeter; solid dots on the carapace (_hartwegi_) were also recorded as one millimeter. Larger ratios are usually derived from measurements of larger individuals. Seemingly, there should be a clinal tendency in ontogenetic variation paralleling the size of ocelli and dependent on it; ontogenetic variation should be least in western populations in which the size of ocelli does not change appreciably with increasing size, and should be greatest in eastern populations in which the ocelli on adult males are larger than those on the carapace of young turtles. It is difficult to demonstrate ontogenetic variation because specimens of corresponding size from the same general area may have ocelli of different sizes. The gradient in size of ocelli is also indicated by specimens from other states. I have the subjective impression that there is least variation in specimens from Michigan (Great Lakes-St. Lawrence River drainage), but this is not clearly shown by Figure 11. [Illustration: FIG. 11. Ratio of diameter of ocellus to length of plastron (OD/PL) in _T. spinifer_ from some states in the upper Mississippi River and Great Lakes drainages. Symbols as in Fig. 3. The size of the ocelli on the carapace gradually increases from Kansas northeastward to Michigan.] Character Analysis _Snout_