106 INTKODUCTION TO CLASSIFICATION. 



are completely walled in by bone, and the Eustachian passages 

 open on the base of the skull. 



There are air passages connected with the tympana in the 

 quadrate and articular bones, and in the supra-occipital. 



The pterygoid unites only with the upper end of the quad- 

 rate ; and the hyoidean apparatus is very simple, and not con- 

 nected directly with the skull. 



There are no clavicles. The pubes are greatly inclined 

 forwards, and remain cartilaginous at their symphysial ends 

 throughout life. 



There are five digits in the fore-foot and four in the hind- 

 foot, but only the three pre-axial (radial and tibial) digits bear 

 nails. 



The teeth are lodged in distinct alveoli, and are confined to 

 the premaxillae, maxillas, and mandible. 



The heart has four completely separated chambers, two 

 auricles and two ventricles, but the right and left aortas are 

 connected by a small aperture immediately above their origin. 

 The sclerotic is not ossified. The ear is provided with a move- 

 able earlid ; and the male has a grooved penis attached to the 

 front wall of the cloaca, the aperture of which is longitudinal. 



The modern Crocodiles, Alligators, and Caimans, and the 

 extinct Teleosauria and Belodonts, form this order. 



2. The LACEKTILIA. An epidermic and dermal exoskeleton 

 is sometimes present, sometimes absent. The dorsal vertebras 

 have proccelous or amphicoelous centra; but their transverse 

 processes are represented by simple tubercles, to which undivided 

 proximal ends of the ribs are attached. There are two, or at 

 most three, vertebras in the sacrum. The presphenoidal region 

 of the skull forms an interorbital septum. 



The quadrate bone is usually movable on the skull, and the 

 pterygoid is almost always connected with its distal end. The 

 hyoidean apparatus is usually large and complicated. The limbs 

 may be well developed, or one pair only present ; or absent. A 

 pectoral arch, consisting of clavicles and more or less ossified 

 coracoscapular cartilages, is always present. The teeth are not 

 lodged in sockets in any recent Lacertilia. 



The heart has three chambers, two auricles and one ven- 



