LACERTILIA AND CROCODILIA. 413 



sluggish animal, but it catches its food, consisting of in- 

 sects, by darting out its long, fleshy, and glutinous tongue 

 an operation which it effects with the most extraordinary 

 rapidity. 



DISTRIBUTION OF LACERTILIA IN TIME. The geological 

 range of the true Lacertilia is not by any means very great, 

 nor, with a single exception, are their remains of much import- 

 ance. The earliest traces of Lizards in the stratified series are 

 found in the fresh-water strata of the Purbeck Beds at the 

 summit of the Jurassic Series. Several small Lizards occur 

 here, and have been described under the names of Nathetes, 

 Macellodon, Saurillus, and Echinodon. The most remarkable 

 fossil Lizard, however, is the Mosasaurus of the Chalk. This 

 gigantic reptile occurs at the very summit of the Cretaceous 

 Series, in what is known as the Maestricht Chalk. The skull 

 is no less than five feet long ; and as the tail and limbs were 

 formed for swimming, there can be little doubt but that Mosa- 

 saurus like the living Amblyrhynchus was aquatic in its 

 habits, and frequented the sea-shore. 



ORDER IV. CROCODILIA. The last and highest order of 

 the living Reptilia is that of the Crocodilia, including the living 

 Crocodiles, Alligators, and Ga vials, and characterised by the 

 following peculiarities : 



The body is covered with an outer epidermic exoskeleton 

 composed of horny scales, and an inner dermal exoskeleton 

 consisting of bony plates or scutes, which may be confined to 

 the dorsal surface alone, or may exist on the ventral surface as 

 well. The bones of the skull and face are firmly united to- 

 gether, and the two halves or rami of the lower jaw are united 

 in front by a suture. There is a single row of teeth, which are 

 implanted in distinct sockets. The centra of the dorsal verte- 

 brae in all living Crocodilia are proccelous, or concave in front, 

 but in the extinct forms they may be either amphiccelous (con- 

 cave at both ends) or opisthoccelous (concave behind). The 

 vertebral ends of the anterior trunk-ribs are bifurcate. There 

 are two sacral vertebrae. False ribs are usually developed in 

 the wall of the abdomen. There are no clavicles. The heart 

 consists of four completely distinct and separate cavities, two 

 auricles, and two ventricles, the ventricular septum as in no 

 other Reptiles being complete. The right and left aortae, how- 

 ever, or, in other words, the pulmonary artery and systemic aorta 

 are connected together close to their origin by a small aper- 

 ture (foramen Panizztz), so that the two sides of the heart 

 communicate with one another. The aperture of the cloaca is 

 longitudinal, and not transverse, as in the Lizards. All the 



