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. 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. ======================+================================================= | Species DEVELOPMENT OF SPUR +--------------+-----------------+---------------- | _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 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_