For quick reference, all the specific and subspecific names proposed for soft-shelled turtles in North America are listed below in alphabetical order (left hand column) with their nomenclatural status as recognized in this paper. The synonyms are listed in the account of the appropriate species or subspecies, and are discussed under the subsection entitled "Remarks." _agassizi_ _Trionyx spinifer asper_ _annulifer_ _Trionyx spinifer spinifer_ _argus_ _Trionyx spinifer spinifer_ _asper_ _Trionyx spinifer asper_ _ater_ _Trionyx ater_ _bartrami_ _Trionyx ferox_ _emoryi_ _Trionyx spinifer emoryi_ _calvatus_ _Trionyx muticus calvatus_ _ferox_ _Trionyx ferox_ _georgianus_ _Trionyx ferox_ _georgicus_ _Trionyx ferox_ _harlani_ _Trionyx ferox_ _hartwegi_ _Trionyx spinifer hartwegi_ _hudsonica_ _Trionyx spinifer spinifer_ _mollis_ _Trionyx ferox_ _microcephalus_ _Trionyx muticus muticus_ _muticus_ _Trionyx muticus muticus_ _nuchalis_ _Trionyx spinifer spinifer_ _ocellatus_ _Trionyx spinifer spinifer_ _olivaceus_ _Trionyx spinifer spinifer_ _spiniferus_ _Trionyx spinifer spinifer_ Variation Aside from qualitative variations and comparisons of patterns of pigmentation the following external measurements (to the nearest millimeter) were used. _Length of plastron_: Maximal straight-line measurement (midventrally), from the anteriormost region of the ventral surface to the posterior end of the plastron; this measurement includes an anterior cartilaginous part. _Length of carapace_: Maximal, straight-line measurement (middorsally), from the nuchal region to the posteriormost region of the free edge of the carapace. _Width of carapace_: Maximal, straight-line measurement between the lateral margins of the carapace. _Plane of greatest width of carapace_: Maximal, straight-line measurement from the posteriormost region of the free edge of the carapace to a point on the middorsal line at the level or plane of the greatest width of the carapace; this measurement and the last two, of course, include the fringing cartilaginous parts of the dorsal bony carapace. _Width of head_: Maximal measurement between the lateral margins of the head. _Length of snout_: Measurement from tip of snout to interorbital region of least breadth. _Diameter of ocellus_: Maximal outside diameter of largest (not conspicuously ovoid or oblong) ocellus on carapace.