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.
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 [460] 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].
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