family had maintained an aquatic mode of life over a long period of geologic time, a view supported by Deraniyagala (1930:1066). Of interest are Stunkard's remarks (1930:214-18) concerning several _Trionyx spinifer_ that were obtained from a commercial supply house and found to be infested with pronocephalid trematodes (_Opisthoporus_ [= _Teloporia_] _aspidonectes_). The closest relatives of that trematode (also recorded from _T. ferox_) live in marine turtles. Possibly, a Mesozoic ancestor of marine and essentially fresh-water soft-shelled turtles harboured ancestors of these trematodes, but possibly the parasites may have transferred relatively recently to their present hosts. Bergounioux (1937:190) judged the Trionychidae to be an ancient group of marine origin. Hummel (1929:770) wrote that the Trionychidae originated in east Asia (the region of most differentiation) in humid climates. Baur (1891:634, 637) pointed out that the dorsal aspect of the skull of the closely related _Carettochelys_ resembles the skull of the Dermatemydidae, Staurotypidae and Kinosternidae; the close relationship of _Carettochelys_ and the Dermatemydidae is also mentioned by Bergounioux (1952:2304) and Hummel (1929:769). Hummel (_op. cit._:771) thought that the Carettochelyidae and "die Chelydroiden" had a common ancestor, and that (_op. cit._:772) the origin of the Trionychidae was older than those two groups. Dunn (1931:109) wrote that the Kinosternidae, Carettochelyidae and Dermatemydidae represented the same general ancestry. Williams (1950:552) has shown the resemblance of the cervical articulations in members of the Chelydridae (including Staurotypinae and Kinosterninae) and the Central American family Dermatemydidae. The consensus of opinion, then, is that the families Trionychidae, Carettochelyidae, Chelydridae and Dermatemydidae are relatively closely related. Genus =Trionyx= Geoffroy, 1809 _Testudo_ Linnaeus (in part), Syst. Nat., Ed. 10, 1:197, 1758; type, _Testudo graeca_ Linnaeus by subsequent designation (Fitzinger, 1843:29). _Trionyx_ Geoffroy, Ann. Mus. Hist. Nat. Paris, 14:1, August, 1809; type, _Trionyx aegyptiacus_ (= _Testudo triunguis_ Forskål) by original designation. _Apalone_ Rafinesque, Atlan. Jour., Friend of Knowledge, Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type, _Apalone hudsonica_ (= _Trionyx spiniferus_ Lesueur) by monotypy. _Mesodeca_ Rafinesque, Atlan. Jour., Friend of Knowledge, Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type _Mesodeca bartrami_ (= _Testudo ferox_ Schneider) by monotypy. _Aspidonectes_ Wagler, Naturl. Syst. Amphib., p. 134, 1830; type, _Aspidonectes aegyptiacus_ Wagler (= _Testudo triunguis_ Forskål) by subsequent designation (Fitzinger, 1843:30).