Certain other families have a distinct longitudinal fold, covered with small granular scales on each side. These are the families _Chalcidæ_, _Holaspidæ_, and the more extensive one of _Zonuridæ_. The _Chalcidæ_ have the head covered with regular many-sided shields, and the lateral fold is indistinct; limbs small and rudimentary, and the hind feet are undivided in the genera _Chalcis_ and _Bachia_, with three tubercles in place of toes in _Microdactylus_, and with four clawed toes in _Brachypus_. Each of these genera is founded on a single species, and all are doubtless peculiar to the New World. The _Holaspidæ_ is also founded on one species only, the _Holaspis Guentheri_, which again is supposed to be South American. It has four well-developed limbs, a double row of plates along the back and upper surface of the tail, and the latter organ is curiously serrated laterally. The _Zonuridæ_ constitute a considerable family, to which some eighteen or twenty genera are assigned, and which present considerable modification of form. The ears are distinct, whereas in the _Chalcidæ_ they are hidden under the skin. The head is pyramidal, or depressed, and covered with regular many-sided shields; eyes with two valvular lids. Limbs mostly well developed, but short in some, and rudimentary, or even wanting in the so-called "Glass-snakes" which constitute the sub-family _Pseudopodinæ_. There is no external trace of them in the North American Glass-snake, _Ophisaurus ventralis_; and in the Old World genus, _Pseudopus_, there is only one pair, posterior, rudimentary, and undivided. These reptiles are long, and serpentiform in shape: whilst in other Saurians the whole skin of the belly and of the sides is extensible, the extensibility is limited in the "Glass-snakes" to a separate part of the skin; and, as Dr. Günther remarks, "the scaly covering of the upper and lower parts is so tight that it does not admit of the same extension as in Snakes and other Lizards; and the _Pseudopus_, therefore, could not receive the same quantity of food in its stomach as those animals, were it not for the expansible fold of the skin running along each side of its trunk." One species of _Pseudopus_, the _P. Pallasii_, inhabits Asia Minor and the south-east of Europe; and there is another, _P. gracilis_, in the Indo-Chinese countries (or those lying eastward of the Bay of Bengal). A second sub-family, _Gerrhonotinæ_, is peculiar to America, and consists of more ordinarily-shaped Lizards, which are ranged in four genera. Together with the _Ophisaurus_, or American Glass-snake, they are the only known _Zonuridæ_ that inhabit the New World. The great mass of this family and all of its most characteristic species are African, and these are arranged by Dr. Gray under the sub-families _Cicigninæ_ and _Zonurinæ_. In the first of these sub-families the tail is smooth, or unarmed, and in the second it is spinous. The Cordules, _Cordylus_, _Zonurus_, &c.,