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. ARTIFICIAL KEY TO NORTH AMERICAN SPECIES AND SUBSPECIES OF THE GENUS TRIONYX 1. Septal ridges present; tubercles on anterior edge of carapace present or absent 2 Septal ridges absent; anterior edge of carapace lacking tubercles or raised prominences 19 2. Plastral area a uniform dark slate or blackish; soft parts of body blackish having large pale marks dorsally; carapace having large black blotches, often fused along margin, on pale background, and many well-defined longitudinal ridges _ T. ferox_, p. 479 Combination of characters not as above; ventral surface whitish, blackish flecks or blotches sometimes present 3 3. Carapace having pattern of white dots, or black ocelli and/or spots; carapace sometimes gritty resembling sandpaper 4 Carapace uniform pale brownish or grayish, or having mottled and blotched pattern, contrasting or not; white dots or tubercles, black ocelli and/or spots may be present; carapace not gritty 10 4. Carapace having pattern of black ocelli and/or spots; numerous, conspicuous whitish spots or tubercles absent 5 Carapace having pattern of white dots that are sometimes surrounded by small black ocelli; small black dots may be interspersed among larger white dots 7 5. Carapace having two or more marginal lines, these often diffuse and interrupted; black spots sometimes ocellate or bacilliform, or interspersed among smaller black dots; postocular and postlabial stripes usually united _spinifer asper_, p. 502 Carapace having only one dark marginal line; pattern of black ocelli or spots; postocular and postlabial stripes usually not united 6