They have a cylindrical rigid body, covered with generally smooth and polished scales; a short strong tail; a short rounded or pointed head with narrow mouth; teeth few in number; small or rudimentary eyes; no abdominal scutes or only narrow ones.
Their body is generally compressed and slender; their broad ventral scutes are often carinate on the sides.
Their body is covered with small scales and the ventral scutes are mostly narrow; the tail tapering; head flat, rather short; and the eyes of small size.
There are no scales developed on any part of the body, but a series of hard and large scutes protects a greater or lesser portion of the sides.
The development of its scutes and spines varies exceedingly, and specimens may be found without any lateral scutes and with short spines, others with only a few scutes and moderately sized spines, and again others which possess a complete row of scutes from the head to the caudal fin, and in which the fin-spines are twice as long and strong as in other varieties.