Certain features of amphisbaenids are common to most or all amphisbaenians. These include: a unique modification of the middle ear in which an elongated structure, the extracol-umella, attaches to the stapedial bone of the middle ear extending forward to attach to tissue along the sides of the face and allowing the reception and transmission of vibrations to the inner ear; reduction or absence of the right lung; an en larged, medial, premaxillary tooth; the periodic shedding of the skin in a single piece; a heavily ossified and robust skull; the absence of eyelids; a forked tongue; and the absence of external ear openings.
The family Amphisbaenidae contains some of the smallest and largest amphisbaenians and even some of the most highly miniaturized reptiles known. One exceptionally tiny amphisbaenid is the African Chirindia, with some species attaining an adult body length of only 4 in (100 mm) and a body diameter of 0.125 in (3 mm). The largest amphisbaen-ian is probably the South American Amphisbaena alba, which attains an adult body length of over 30 in (800 mm). In general, amphisbaenians show little or no sexual dimorphism in body size. All amphisbaenids are completely limbless, but some retain internal vestiges of the pectoral and pelvic girdles. In some species, a small eye is visible under a translucent head scale, but in others the eye is not visible at all. Amphisbaenids exhibit a variety of cranial shapes (keel-headed, round-headed, shovel-headed) and a generally cylindrical body shape. In some species the snout is conical and in others it is blunt. The tail is always short but may exhibit
a variety of shapes, including rounded and blunt-tipped, pointed, or dorsoventrally flattened. Most amphisbaenid species are capable of autotomizing the tail, but none can regenerate a new tail. All amphisbaenids have pleurodont dentition, with varying numbers of teeth occurring on the tooth-bearing elements of the skull.
Many species are pallid in appearance, presumably related to their subterranean existence. However, some species exhibit striking patterns of coloration, including dark checkerboard patterns on light backgrounds. Still others are a solid brown, yellow, or grey dorsally, with a paler underside.
Beyond those easily recognizable features, amphisbaenids are also characterized by a number of internal conditions, including the presence of pelvic vestiges in all species, the absence of a sternum, and a heavily ossified skull.
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