Most skulls of _ferox_ differ from those of _muticus_ and _spinifer_ in having the greatest width at the level of the quadratojugal (as do some _T. s. asper_; see account of that subspecies). In the skull, _ferox_ resembles _spinifer_ but differs from _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. ferox_ resembles _muticus_ but differs from most individuals of _spinifer_ in lacking a well-developed opisthotic-exoccipital spur. _T. ferox_ resembles _spinifer_ but differs from _muticus_ in having the epiplastron bent at approximately a right angle; _ferox_ differs from both _muticus_ and _spinifer_ in lacking a callosity on the epiplastron and probably in the more frequent fusion of the hyoplastra and hypoplastra. _Remarks._--The early taxonomic history of _Trionyx ferox_ has been discussed in detail by Stejneger (1944:27-32), who explained that Dr. Alexander Garden of Charleston, South Carolina, sent a description and specimen of _T. ferox_ to Thomas Pennant, and at the same time sent another specimen with drawings to a friend, John Ellis, in London. Pennant presented one of the specimens and drawings and the description to the Royal Society of London in 1771; the description was published in 1772 and included Garden's drawings. Because two specimens were involved the possibility exists that the description (text, drawings and type specimen) is a composite based on two specimens.