. Water reptiles of the past and present . uborders, the Lacertilia or lizards;the Dolichosauria or long-necked lizards of the past; the Mosa-sauria, or extinct swimming lizards; and the Serpentes or Ophidia,the snakes. It matters very little which classification one acceptsso long as it is remembered that the first three groups are closelyrelated to each other. 140 WATER REPTILES OF THE PAST AND PRESENT LIZARDS Popularly a lizard is any four-legged reptile covered withscales, but such a definition is not strictly correct, since somelizards are legless and some other four-legged reptiles are c
. Water reptiles of the past and present . uborders, the Lacertilia or lizards;the Dolichosauria or long-necked lizards of the past; the Mosa-sauria, or extinct swimming lizards; and the Serpentes or Ophidia,the snakes. It matters very little which classification one acceptsso long as it is remembered that the first three groups are closelyrelated to each other. 140 WATER REPTILES OF THE PAST AND PRESENT LIZARDS Popularly a lizard is any four-legged reptile covered withscales, but such a definition is not strictly correct, since somelizards are legless and some other four-legged reptiles are coveredwith horny scales, notably the tuatera or Sphenodon of NewZealand, a reptile long classed with lizards, but now known tobelong to quite a different order. Bearing in mind those charactersgiven as characters of the order, it will be necessary to mentiononly those distinguishing the lizards from the snakes. It is true that the great majority of lizards have four legs, whilethe snakes are always functionally legless, but there are some. Fig. 65.—Iguana. (By permission of the New York Zoological Society) lizards, like the glass snakes and the amphisbaenas, or slow lizards,which are quite legless and there are some snakes which have smallbut functionless hind legs. As usual, more important differencesare found in the skull. The brain-case in all snakes is surroundedon all sides by bone, for the better protection of the brain, with thehead resting quite prone on the ground. The brain of the lizards,for the most part, is protected on the sides and in front by a simplemembrane. Nearly all lizards have movable eyelids, while snakesdo not; snakes have a single lung, and a protrusible tongue, whichvery few lizards possess; and the lower jaws in front are unitedin the snakes by a ligament only. Notwithstanding these differ-ences, the snakes and lizards are closely related animals, and must SQUAMATA 141 have come from a common ancestry; among all reptiles the knowngeological history
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