| Month | Gill-net days | Number of turtles | Gill-net days per turtle |
| July | 835 | 213 | 3.9 |
| August | 816 | 199 | 4.6 |
| September | 743 | 42 | 17.7 |
| October | 1661 | 82 | 20.3 |
| November | 1322 | 48 | 27.5 |
| December | 864 | 5 | 172.8 |
Dr. Virgil Dowell, while making fishery studies two miles east of Willis, Marshall County, Oklahoma, caught, on the average, 1.5 turtles per day. Of 75 turtles collected from July 1 through October 18, 1953, 66 were Trionyx (spinifer and muticus), five were Graptemys and four were Pseudemys scripta. No more than two gill nets were used simultaneously. The nets were moved from time to time and varied in dimensions, but those used most of the time were 200 feet long and eight feet deep with a 3-inch mesh.
The few captures by Houser probably resulted from long-continued trapping in one place; the gill nets were not moved in the entire six-month period or for some time previously. Breckenridge (1955:6) commented on the sedentary nature of spinifer (in Minnesota) and quoted a professional turtle trapper as stating that "after a section of a river has been trapped heavily for softshells, little success can be expected in that area for as much as three or four years thereafter." Both Houser's and Dowell's data indicate a higher percentage of soft-shelled turtles collected than any other species. The number caught probably depends, at least partly, on the food habits of the species and is influenced by the enmeshed fish, which, serving as a food source, attract the turtles.
Materials and Procedures
In the course of this study I examined 1849 soft-shelled turtles, including some incomplete alcoholic or dried specimens, such as those represented only by skulls or by other osteological material. Material was examined from each of the collections named below (except KKA), and these are mentioned in the text by the following abbreviations:
| AMNH | American Museum of Natural History |
| ANSP | Academy of Natural Sciences, Philadelphia |
| BCB | Bryce C. Brown, private collection, Baylor University |
| CM | Carnegie Museum |
| CNHM | Chicago Natural History Museum |
| INHS | Illinois Natural History Survey, University of Illinois |
| KKA | Kraig K. Adler, private collection, data in letter dated January 8, 1960 |
| KU | Museum of Natural History, The University of Kansas |
| LSU | Louisiana State University |
| MCZ | Museum of Comparative Zoology, Harvard College |
| MSU | The Museum, Michigan State University |
| NHB | Naturhistorisches Museum Basel, Switzerland |
| OU | University of Oklahoma Museum, Division of Zoology |
| SM | Strecker Museum, Baylor University |
| TCWC | Texas Cooperative Wildlife Collection, Texas Agricultural and Mechanical College |
| TNHC | Texas Natural History Collection, The University of Texas |
| TTC | Texas Technological College |
| TU | Tulane University |
| UA | University of Alabama |
| UI | Museum of Natural History, The University of Illinois |
| UMMZ | Museum of Zoology, The University of Michigan |
| USNM | United States National Museum |
| WEB | William E. Brode, private collection, Mississippi Southern College |
| WTN | Wilfred T. Neill, private collection |
External measurements (listed under the section, "Variation") were taken by the writer by means of a Vernier caliper or a steel tape. Measurements of the skulls are in millimeters and tenths as taken by the writer with dial calipers. Partial wrinkling of the carapace at the edges of some specimens causes some error in measurements; consequently, length of plastron is used as the measurement of reference.
Scattergrams based on external measurements were constructed. Some demonstrate considerable ontogenetic variation. An inspection of the scattergrams indicated regressions essentially linear in nature, but sometimes occasioned an arbitrary separation of samples into size groups to show ontogenetic variation; no secondary sexual differences could be discerned. Several ratios were developed from the measurements. The data correspond to the regression [438] model 1A in "Statistical Methods" (Snedecor, 1956, sec. 6.13); consequently, the sample ratios indicate the slope of regression and are useful in comparisons. Sample-means and their estimated standard errors are compared graphically to show general trends in proportional characters. Comparisons of means and standard errors indicate statistical significance between populations if the sample-means plus or minus twice their standard errors do not overlap, but this method of comparison is valid only when comparing two samples (Pimentel, 1959:100).
In the section on "Variation," general features applicable to all kinds of soft-shelled turtles are discussed under the following headings: secondary sexual, ontogenetic, and geographic; individual variation is mentioned in accounts of species and subspecies. In the section "Character Analysis" external and osteological characters having taxonomic significance are discussed.
Vernacular names follow, as closely as possible, those recommended by the Committee on Herpetological Common Names (1956). The synonymy of each monotypic species or subspecies begins with the name as given in the original description. The second entry is the name-combination herein applied to the taxon. Other entries are first usages, in chronological order, of other names (synonyms) that have been applied to the taxon in question. Next, the type is briefly discussed followed by the "Range" defined in general geographic terms, and, when appropriate, in terms of river drainage systems. "Diagnosis" includes a combination of characters that facilitates quick identification. In polytypic species, the diagnosis of a subspecies is designed only to distinguish it from other subspecies of that species. The comments included under the subsection entitled "Description" pertain to individuals from an area where the taxon is most clearly differentiated. Because osteological characters are significant only at the specific level, they appear under the accounts of each species (excluding ater). Proportional characters as given in the "Diagnosis" are only in general terms; more specific data are set forth in the subsection, "Description" or in the various text figures, mostly in the section on "Variation," page 445. Proportions pertaining to the species muticus were derived only from the nominal subspecies, and appear under the account of the species. A subsection "Variation" under the accounts of some subspecies includes information concerning principally individual variation and coloration; because color is not considered to be of major taxonomic importance, color terms are used without reference to any standard color guide. The subsection "Remarks" follows the section on "Comparisons," and may include comments on nomenclature, intergradation and other information related to the distribution or taxonomy of the subspecies.