598 CHORDATA. 



require periodic removal, the horny layers are cast yearly and re- 

 placed by new. During this periodic molting, which recalls that 

 of arthropods, the animals are sickly and apt to die in captivity. 



All Squamata are characterized by the slenderness of the 

 cranial bones (fig. 619, 626, 627), which, especially in the Lacertilia, 

 incompletely close in the cranium. The quadrate is movable, 

 and the squamosal is intercalated between it and the cranium. A 

 hard palate is lacking, and the choanse, as in the amphibia, lie far 

 forward (fig. 619, Ch). There is a wide gap in the partition 

 between the two ventricles of the heart. 



Sub Order I. LACERTILIA (Saurii). The lizards are usually distin- 

 guished from, the snakes by the possession of limbs, but a few forms, 

 undoubted lizards, like the glass snakes and Amphisbsenaa. lack limbs. 

 These are distinguished by the existence of the scapula and the iliac bone 

 united to the vertebra, and especially by the presence of a sternum, which 

 never occurs in snakes. In the skull is a peculiar bone (lacking only 

 in Chameleons and Amphisbsenae), found nowhere else, the epipterygoid 

 (fig. 626, co); it reaches from the pterygoid to the parietal, and from its 



fffrffr 



FIG 627. Skull of rattlesnake. (From Boas.) F>; frontal; ft, hypmandibular (09111- 

 mella); MX, maxillary: iV, nasal: Os, supraoccipital; Fa, parietal; Pal, palatine; 

 P/, postfrontal; PC/, pref rontal ; Pt, pterygoid ; Px, premaxilla ; Q, quadrate; 3g, 

 squamosal; 2V, transversum; 1, dentary; 3, articulare. 



slender shape is sometimes called columella, but is not to be confounded 

 with the true columella of the ear. The bones of the jaws are firmly united, 

 so that the mouth has no special capacity for opening widely. The jugal- 

 quadratojugal arch is present. 



In external appearance the presence of eyelids, nictitating membrane, 

 tympanic membrane, and Eustachian tube are noticeable, these being 

 absent only in the Amphisba3na3. In the Ascalabota?, as in snakes, the lids 

 grow together, forming a transparent covering over the eyes. Fossil 

 lizards are rare, but the group dates back to the cretaceous. 



Section I. ASCALABOT^E (geckos). Skeleton incompletely ossified, noto- 

 chord persistent, amphicoele vertebrae; skin granular rather than scaly, 

 usually adhesive discs on the toes by which they climb vertical surfaces or 

 can walk upon ceilings. Two hundred species. Phyllodactylus* 



