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Two subspecies are recognized. Geoffroy's side-necked turtle, Phrynops geoffroanus geoffroanus (Schweigger, 1814), occurs in central and southern Brazil, Paraguay, northeastern Argentina, and Uruguay. It has only a medial vertebral keel, and its intergular scute is shorter than its distance from the abdominal scute. P. g. tuberosus (Peters, 1870), Peters' side-necked turtle, occurs in the Guianas and eastern Brazil southward to Bahia. A population of this subspecies also occurs in Colombia, but Pritchard (1979) feels it may represent a different subspecies. P. g. tuberosus has a medial vertebral keel flanked on each sideby a lateral keel formed by a series of raised knobs along the dorsal surface of each pleural scute, and its intergular is usually longer than its distance from the abdominal. Lamar and Medem (1982) have noted that those Colombian turtles referred to this race differ considerably from the nominate form. Their status needs clarification. The southernmost populations of P. geoffroanus have been designated as a separate species, P. williamsi Rhodin and Mittermeier, 1983.
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