LIZARDS. 405 



Order III. — LACERTILIA. 



To this order belong the lizarJs, wliieh in:iy be dctiiu'd as all iiciw existing reptiles 

 having a pectoral girdle and sternum, and, as a rule, four limbs. A tympanic cavity 

 is usually present, and the eyes, with a fe\y exceptions only, are ])royided with lids. 

 The bones of the jaws and head do not allow that expansiljility generally character- 

 istic of snakes, and a still further departure is made froni this group in tlial the lizards 

 are provided with a iirinarj' bladder. 



The lizards are generally of au elongated form, and snake-like, some carrying the 

 resemblance still furtlier by having the limbs reduced to rudiments, or externally en- 

 tirely absent. The limbs, when present, are seldom sufficiently strong to support the 

 body from the ground, and are lience used more as pushing organs, though the cha- 

 meleons have them designed for grasj)ing, the geckos modified into sucking disks, by 

 wliich they can ascend perpendicular walls, some of tlie iguanas for swimming, and 

 yet others for digging. It .will thus be seen that the animals are designed for differ- 

 ent modes of life. While some lizards are terrestrial, and have the lindjs poorly 

 de\eloped or even absent, others are arboreal, and, like the arboreal serpents, are spe- 

 cially modified and protectively colored. It is in this order, however, that we first 

 find reptiles designed for au aerial, or partially aerial, life. The shoulder girdle is 

 always j)resent, and the shape of the clavicle is of considerable taxonomio value. The 

 sternum is absent in a single genus, Amphlsbcena. The ribs are generally present, 

 and extend fi-om the anterior cervical to the lumbar vertebraj. These ribs, in forms 

 like Draco and Liolephis, are the chief organs of sujijJort for the wing-like expansions 

 of the sides of the body. 



The structure of the skull is particularly interesting, though complex. On its 

 peculiarities has been based the only natural classification. The cranium proper, that 

 portion of the skull enclosing the brain, is relatively small ; it does not extend to the 

 orbital region, and is protected in front by a vertical curtain, the membranous inter- 

 orbital septum. The bones of the jaw are connected with those of the cranium by 

 the intervention of an arch, the zjgomatic, made up of the malar, postorbital, and 

 squamosal bones. The quadrate is large and much more firmly attached than in either 

 of the jirevious orders. The rigidity with which the bones are united is worthy of 

 notice, and particularly interesting when compared with the loosely articulated facial 

 bones of the serpents. The lower jaw is incapable of lateral expansion, either by 

 means of an elastic symphysis, or by a medial joint. 



The dentition is peculiar, and, to a certain extent, characteristic. Much more 

 variety as regards general structure, mode of insertion, and position of teeth, is pre- 

 sented than in the previous orders, though these peculiarities are of only secondary 

 value in classification. In many families there is an interesting distinction between 

 Old and New World forms ; the former having the teeth planted along the ridge of 

 the jaw are termed acrodont, while the latter have them merely appressed to the 

 inside, and are plurodont. A ])eculiar anomaly is presented by the American genus, 

 Teius, the teeth in the young being j)lurodont, and, as age j)roceeds, by a growth of 

 the bone of the jaw around their bases gradually becoming acrodont. 



Though the shape of the tongue is very variable, and, to a certain extent, charac- 

 teristic, its covering is of far more im])ortance to the systematic zoologist. Though 

 the lower families and the degraded Amphisbajnas have the eyes, like the serpents, 



