posterolaterally to cover the entire surface of the epiplastron as it may in _muticus_ (Fig. 17b). The epiplastron of a _spinifer_ (KU 2826) has a medial callosity and another on the right posterolateral projection; three separate callosities occur on the epiplastron of MCZ 46615. The last specimen mentioned, a large, stuffed female, possesses a round, intercalary bone that tends to occlude the posteromedial vacuity. Seemingly, the callosity on the epiplastron appears prior to those on the preplastra; I have not seen any plastra having callosities on the preplastra and lacking a callosity on the epiplastron. I have not noted callosities on the preplastra or epiplastron of specimens of _ferox_. The callosities on the plastral bones are sculptured; small, recently formed callosities on the preplastra and epiplastron lack sculpturing. The pattern of sculpturing on the plastral bones as well as that of the carapace is generally of anastamosing ridges. I am unable to discern any differences in pattern of sculpturing between the three American species. Stejneger distinguished adult specimens of _ferox_ from the other American species by the coarseness of the sculpture of the bony callosities (1944:24) and of the bony carapace (_op. cit._:32). The sculpturing on the plastral callosities and carapace seems to be correlated with size; larger specimens (_ferox_) have coarser sculpturing than do smaller specimens (_muticus_). Stejneger also mentioned that the sculpturing on many specimens of _ferox_ is specialized into prominent, longitudinal welts (_loc. cit._); these welts occur also on the carapace of _spinifer_. On the basis of the osteological characters examined by me, _T. muticus_ is distinguished from _spinifer_ and _ferox_ by a number of characters (plastron and especially skull) whereas the species _spinifer_ and _ferox_ are not easily distinguished from one another. Composition of the Genus _Trionyx_ in North America Analysis of the characters previously mentioned and their geographic distribution permits the recognition of ten taxa, comprising four species and eight subspecies. Two subspecies, _T. spinifer_ pallidus and _T. s. guadalupensis_ are described as new. The four species and the included subspecies here recognized are: _Trionyx ferox_ _Trionyx spinifer spinifer_ _hartwegi_ _asper_ _emoryi_ _guadalupensis_ _pallidus_ _Trionyx ater_ _Trionyx muticus muticus_ _calvatus_