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 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 [476] 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
The following key is designed to permit quick identification of living individuals; therefore, ratios and osteological characters are avoided as much as possible in favor of other characters that are the least variable and most "typical." Because there is considerable variation correlated with sex and size, each taxon occurs in the key in more than one couplet. Large females having mottled and blotched patterns will be the most difficult to identify. The characters listed should be used in combination because one character alone may not be sufficient; it is advisable to read both choices of each couplet. The text, figures and illustrations should be consulted for final identification.