g. muticus, KU 2840, White River, DeValls Bluff, Prairie County, Arkansas.
h. muticus, USNM 115939, Mississippi.
i. muticus, USNM 54734, Mississippi River, Fairport, Muscatine County, Iowa.
Most variation concerns the number of neurals and pairs of pleurals, and their arrangement posteriorly (H. M. Smith, 1947:121, table; Stejneger, 1944:18). [Table 5] shows the frequency of occurrence of the number of neurals, pairs of pleurals, and the separation or contact of the seventh pair of pleurals; figure 16 illustrates some of the configurations of these plates posteriorly (e, g, and i not included in [Table 5]). The eighth pair of pleurals is reduced or absent (Loveridge and Williams, 1957:417). Eight neurals and eight pairs of pleurals occur in all three species. The seventh pleurals may contact each other in all three species, and their separation has been observed only in the species spinifer and muticus. Seven neurals and contact of the seventh pair of pleurals, or eight neurals and separation of the seventh pair of pleurals from each other occurs with approximately equal frequency in the species muticus. T. ferox and spinifer most often have seven neurals, seven pairs of pleurals, and the seventh pair of pleurals in contact. Stejneger (loc. cit.) mentioned a specimen in MCZ having nine neurals; I recorded nine neurals for USNM 54734 ([Fig. 16i]) for which Stejneger (loc. cit.) recorded eight. AMNH 57384 (ferox) has a small eighth pleural on the left side only, and USNM 115939 (muticus) has an eighth pleural only on the right side ([Fig. 16h]). Anomalous conditions observed included: an accessory bone between the first and second pleurals on the right side that contacts the first and second neurals in USNM 54733, (muticus); only six neurals in USNM 95193 (spinifer); a small accessory bony element between the first and second neurals in AMNH 57383 (ferox); and, only six pleurals (second and third fused) on the right side in USNM 54734 (muticus).
Table 5. Frequency of Occurrence of Number of Neurals, Pairs of Pleurals, and Separation or Contact of the Seventh Pair of Pleurals Among Species of American Soft-shell Turtles
| Number | Contact (+) or separation (-) of seventh pair of pleurals | Species | |||
| Neurals | Pairs of pleurals | ferox (16) | spinifer (60) | muticus (34) | |
| 7 | 7 | + | 9 (56%) | 50 (83%) | 13 (38%) |
| 7 | 8 | + | 5 (31%) | 2 (3%) | 2 (6%) |
| 8 | 7 | + | 2 (13%) | 3 (5%) | 3 (9%) |
| 8 | 8 | + | ⋯ | 4 (7%) | 2 (6%) |
| 8 | 7 | - | ⋯ | 1 (2%) | 14 (41%) |