Table 3. Variation in Position of Greatest Width of Skull of North American Species of the Genus Trionyx (excluding ater). The Number of Specimens Examined (in Parentheses) Follow the Specific Names.
| Position | Species | ||
| ferox (36) | spinifer (47) | muticus (14) | |
| Squamosal | 7 (19%) | 35 (74%) | 11 (79%) |
| Quadratojugal | 26 (72%) | 7 (15%) | 1 (7%) |
| Quadrate | 2 (6%) | ⋯ | 2 (14%) |
| Squamosal and quadratojugal of same width | 1 (3%) | 5 (11%) | ⋯ |
Supraoccipital Spine
The ventral surface of the supraoccipital spine in muticus lacks a medial ridge, and gradually increases in width anteriorly, so that it is widest proximally in the region of the roof of the foramen magnum. In ferox and spinifer, the ventral surface, usually having a medial ridge, is narrow and of the same width throughout its length or somewhat flared distally. The ventral surface of the supraoccipital spine, which is widest proximally in muticus, is always narrow proximally in ferox and spinifer. The ventral surface of the supraoccipital spine of one skull of spinifer, USNM 91311, differs little from that of muticus.
Foramen Magnum
The shape of the foramen magnum is generally rhomboidal in spinifer and ferox; the ventral angle is semicircular, the lateral angles obtuse, and the dorsal angle more acute. The shape of the foramen magnum in muticus is ovoid, higher than wide; the sides are evenly rounded.
Opisthotic-Exoccipital Spur
Skulls of spinifer normally have the fenestra postotica partly restricted by a medially-slanting, descending spur from the roof of the fenestra postotica; the spur incorporates the suture between the exoccipital and opisthotic and includes parts of those two bones. On one skull (KU 2824) the spur is displaced more medially and does not incorporate the opisthotic. The descending spur contacts the pterygoid ventrally forming a complete bony strut traversing the fenestra postotica in some skulls (KU 2228, 2666, 2762, TU 15423, MCZ 46621, TU 15415, right side only). The fenestra postotica on skulls of ferox and especially muticus is not normally restricted by an opisthotic-exoccipital spur.
Often the spur is reduced and indicated by a smooth projecting ridge. Sometimes the spur or ridge is absent on skulls of spinifer, and I have seen no well-developed spur on a skull of muticus. The development of the spur is not due to ontogenetic variation. There is some variation in development of the spur on either side of the skull; two skulls of ferox have the combination ridge/absent, and two of spinifer have the combinations ridge/spur and spur/absent. The frequency (based on counts of individual skulls) and the degree of development of the spur among the three species is indicated in [Table 4].