The total of 159 skulls examined by me include 80 of spinifer, 50 of ferox, and 29 of muticus. There are no secondary sexual differences between skulls of corresponding size, except in agassizi-form skulls mentioned under the account of T. s. asper, and possibly in ferox. Most, and possibly all, of the skulls of muticus having a basicranial length of 40.0 millimeters or more, and those of spinifer exceeding 50.0 millimeters must represent females (by correlation of known maximum size of males with greatest width of head, which is, in turn, compared with the greatest width of skull and corresponding basicranial length).
Fig. 15. Skulls of Trionyx spinifer hartwegi (left, a-d, KU 2757), and Trionyx muticus muticus (right, e-h, KU 1870). Dorsal views, a (× 1/2), e (× 3/4); occipital views, b (× 5/6), f (× 1); lateral views, c (× 1/2), g (× 3/4); ventral views, d (× 1/2), h (× 3/4).
a., alveolar surface of upper jaw
aq., articular surface of quadrate
ex., exoccipital
fp., fenestra postotica
fm., foramen magnum
if., intermaxillary foramen
ic., internal choana
mx., maxilla
mxb., maxillary bridge
oc., occipital condyle
op., opisthotic
ope., opisthotic-exoccipital spur
opw., opisthotic wing
pmx., premaxillaries (fused)
pt., pterygoid
q., quadrate
qj., quadratojugal
sq., squamosal
s., supraoccipital spine
tc., tympanic cavity
Measurements used include basicranial length (occipital condyle to tip of upper jaw), greatest width (variable in position), greatest width of alveolar surface of maxilla (taken at level immediately posterior to anterior margin of internal choanae), greatest length of internal choanae, and least breadth of maxillary bridge (separating internal choanae and intermaxillary foramen). One ratio developed from the measurements was greatest length of internal choanae/least breadth of maxillary bridge, hereafter referred to as IC/MB. This ratio is discussed under the account of T. s. asper.
Greatest Width
The position or level on the skull where the greatest width ([Table 3]) occurs is of some diagnostic value in distinguishing the skulls of ferox from spinifer and muticus. Skulls of ferox usually are widest at the level of the quadratojugal (immediately in front of tympanic cavity), whereas skulls of spinifer and muticus usually are widest slightly more posteriorly at a level on the squamosal immediately behind the tympanic cavity. Occasionally the width at the level of the quadratojugal and squamosal is the same, or the greatest width of skull may be ventrad between the quadrates, which are slightly flared laterally. The latter condition possibly is most prevalent in muticus.