Table 4. Frequency and Degree of Development of Opisthotic Exoccipital Spur of North American Species of the Genus Trionyx (excluding ater). The Number of Specimens Examined (in Parentheses) Follow the Specific Names.
| Development of Spur | Species | ||
| ferox (43) | spinifer (68) | muticus (29) | |
| spur (well-developed) | 1 (2%) | 45 (66%) | ⋯ |
| ridge (reduced) | 7 (16%) | 20 (30%) | 1 (3%) |
| absent | 35 (82%) | 3 (4%) | 28 (97%) |
Loveridge and Williams (1957:415, footnote) cited Siebenrock who mentioned a descending process of the opisthotic in Dogania (= Trionyx) subplana and Trionyx sinensis. I have not seen an ascending process of the pterygoids on skulls of American softshells as described by Loveridge and Williams (op. cit.:414, 429, fig. 54) for Lissemys, Cyclanorbis, Cycloderma and some Trionyx triunguis.
Opisthotic Wing
This term refers to the laterally directed, posterior part of the opisthotic that is visible in occipital, lateral and ventral views. In ventral view the opisthotic wing is most easily seen and is wider in muticus than in spinifer or ferox. In muticus the distal part is truncate, whereas in ferox and spinifer, it is more tapered and gently rounded, although somewhat unevenly flared [469] medially. Also there is more of a downward curvature (in ventral view) of the opisthotic wing in muticus than in ferox or spinifer; consequently the tip of the wing in muticus is often just visible in dorsal view (on lateral side of squamosal), certainly in lateral view. The distal part or tip of the opisthotic wing is not visible in dorsal view on skulls of ferox or spinifer.
Articular Surface of Quadrate
The ventral surface of the quadrate that articulates with the mandible is composed of a lateral condyle and a medial articular surface. The condyle and medial articular surface are separated by a furrow. On skulls of ferox and spinifer the lateral condyle, which is not conspicuously tapered posteriorly, is slightly larger than the medial articular surface, and the furrow is shallow. On skulls of muticus, the lateral condyle is conspicuously tapered posteriorly, is slightly smaller than the medial articular surface, and the furrow is deep.
Contact of Maxillaries Above Premaxillaries
The contact of the maxillaries above the premaxillaries is of diagnostic value in distinguishing skulls of ferox and spinifer from those of muticus. I have seen no skulls of muticus on which the maxillaries were in contact, and no skulls of ferox on which the maxillaries were separated. Stejneger (1944:19), however, reported a skull of muticus (USNM 102677) having the maxillaries in contact. Maxillaries are in contact (sometimes just barely) in 65 of 74 skulls of spinifer (88%); the premaxillaries are separated on nine skulls (12%).
Carapace