_T. spinifer_ is intermediate in size between _ferox_ (larger) and _muticus_ (smaller); the maximum size of the plastron in adult males is approximately 16.0 centimeters (14.0, _muticus_; 26.0, _ferox_), and of females, 31.0 centimeters (21.5, _muticus_; 32.5, _ferox_). The head for all subspecies of _spinifer_ is proportionately narrower than in _ferox_ but wider than in _muticus_. In the skull, _spinifer_ more closely resembles _ferox_ than _muticus_, but differs from both _ferox_ and _muticus_ in usually having a well-developed opisthotic-exoccipital spur. Skulls of _spinifer_ resemble those of _muticus_ but differ from those of _ferox_ in being widest at the level of the squamosal. Skulls of _spinifer_ resemble those of _ferox_ but differ from those of _muticus_ in having the 1) ventral surface of the supraoccipital spine narrow proximally, and usually having a medial ridge, 2) foramen magnum rhomboidal, 3) distal part of opisthotic wing tapered, 4) lateral condyle of articular surface of quadrate not tapered posteriorly, and larger than medial articular surface, and 5) maxillaries in contact above premaxillaries. _T. spinifer_ resembles _ferox_ but differs from _muticus_ in having the epiplastron bent at an approximate right angle. _T. spinifer_ differs from _ferox_ in having an epiplastral callosity, and from _muticus_ in that the callosity does not cover the entire surface of the epiplastron. The hyo-hypoplastral suture is present more often in _spinifer_ and _muticus_ than in _ferox_. _Remarks._--Gray (1869:221) proposed the generic name _Callinia_ as a new name for _Aspidonectes_ as understood by Agassiz (1857:403). Gray referred _Trionyx spiciferus_ (= _spiniferus_) Lesueur to _Callinia_. Stejneger (1907:514) designated _Trionyx spiniferus_ Lesueur as the type species of _Callinia_. If _Trionyx spiniferus_ Lesueur is considered to be generically distinct from other soft-shelled turtles, _Callinia_ Gray, 1869, is available as a generic name with _Trionyx spiniferus_ Lesueur, 1827, as the type species by subsequent designation.