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. ================+================================================= | Species POSITION +--------------+-----------------+---------------- | _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.