Wagler, 1830
South American snake-necked turtles
Recognition
Hydromedusa is the genus of two semi-aquatic South American turtles most closely resembling the long-necked Australian chelids; its neck is longer than its carapacial vertebral column. The carapace is flattened and may bear conical protuberances on the vertebrals and pleurals throughout life. There are six to nine neural bones, and in one species, H. maximiliani, the nuchal bone lies behind the anterior peripherals and does not reach the carapacial rim. The 7th or the 7th and 8th pairs of costal bones meet at the midline. The large cervical scute lies behind the anterior marginals and does not extend to the carapacial rim. The plastron is extensive, lacks a hinge, and has a large intergular scute separating the gulars. Its axillary buttresses are strong, but the inguinal buttresses are weak. The skull is long and flattened with the prefrontal bones meeting at the midline so that the separate frontal bones are exposed anterior and posterior to the point of contact. A rather slender temporal arch is formed by the squamosal and supraoccipital bones. The dorsal part of the parietal is absent, and its lateral edges are reduced and tapered posteriorly. The parietal contacts the supraoccipital, but there is no vomer-palatine contact. There is a large bony opening formed by the reduction of the palatines. Only limited contact occurs between the basisphenoid and quadrate bones. The jaws lack triturating ridges. A valvelike flap occurs at the corner of the mouth in H. maximiliani. No chin barbels are present. The toes are fully webbed and each foot bears four claws.
Species identification
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