Much importance has been credited to the fusion (no suture) or separation (suture present) of the hypoplastra and hyoplastra. The fusion of these bones distinguishes the genera _Lissemys_, _Cyclanorbis_ and _Cycloderma_ from _Trionyx_, _Pelochelys_, and _Chitra_ (Siebenrock, _op. cit._:815, 817; Loveridge and Williams, 1957:415). This character is also one of the criteria used by Hummel (1929: 768) in his erection of the two subfamilies Cyclanorbinae (= Lissemyinae) and Trionychinae. In my examination of specimens this character, unfortunately, was not given full attention. I have noted the fusion of the hypoplastra and hyoplastra in KU 1878 (_muticus_, right side only), KU 2219 (kyphotic _spinifer_), KU 16528 (_ferox_) and KU 60121 (_ferox_). Dr. Ernest E. Williams informs me in a letter of November 17, 1959, that of six specimens of _ferox_ in the MCZ, the hyoplastra are fused with the hypoplastra in three (54689-90, 54686). I suspect that these bones in the three American species of the genus _Trionyx_, especially in _ferox_, fuse more often than is supposed. In _muticus_ the constricted part of the hyoplastron and hypoplastron is wider anteroposteriorly than in _spinifer_ or _ferox_ (Fig. 17). The three American species have on the hyoplastra, hypoplastra, and xiphiplastra well-developed callosities, which enlarge with increasing size. The medial borders of the hyoplastral and hypoplastral callosities in larger specimens are rounded and closely approximated, often touching, as do the callosities of each xiphiplastron; seemingly, the callosities are relatively larger in _muticus_ than in _spinifer_ and _ferox_. I have seen one adult male _muticus_ (KU 41380) that lacked median fontanelles or vacuities owing to the contact of the plastral elements (as viewed through overlying skin, alcoholic specimen). The bony plastron (approximately 9 cm. in maximal length) of a small _muticus_ (KU 19460) resembles the plastron of larger individuals of _muticus_ in having well-developed hyoplastral and hypoplastral callosities that are closely approximated medially. Large individuals of _muticus_ usually have small, ovoid callosities on the preplastra, and a well-developed, angular callosity on the epiplastron (Fig. 17a). Siebenrock (_op. cit._:823) suggests that the presence of callosities on the preplastra and epiplastron of _muticus_ is subject to individual variation. I can not substantiate or dispute the supposition of Baur (1888:1122), Siebenrock (1924:193) and Stejneger (1944:12, 19) that the callosities are larger in males of _muticus_ than in the females. Some individuals of _spinifer_ have seven plastral callosities (KU 2842) as does _muticus_, but the callosities on the preplastra and epiplastron are less frequent and less well-developed in large specimens of _spinifer_ than in _muticus_. The small epiplastral callosity in _spinifer_ is located at the medial angle and does not extend