110 INTRODUCTION TO CLASSIFICATION. 



any exoskeleton. Their dorsal vertebra? are amphicoelous, with 

 crocodilian transverse processes. 



The sacrum is large, and formed by the ankylosis of many, 

 sometimes six, vertebrae. 



The cranial and upper facial bones are all firmly fixed 

 together, in which respect, and in the conversion of the jaws 

 into a kind of beak, which appear to have been sheathed in 

 horn, they resemble the Chelonia. 



Teeth seem to have been absent in some species, in 

 others there was a pair of great tusks, with persistent growth 

 implanted in the upper jaw. 



The pectoral and pelvic arches were very strong, and the 

 limbs well developed and fitted to support the massive body on 

 land. 



4. The PTEROSAURIA. The flying Lizards of the Mesozoic 

 epoch are not known to have possessed any exoskeleton. 



The dorsal vertebrae are proccelous, with crocodilian transverse 

 processes: ossified sternal ribs, and splint-like, false, or exo- 

 skeletal, abdominal ribs are present. 



There is a broad sternum, with a median keel or crest. 



The skull is in many respects very bird-like, but the jaws 

 carried teeth, implanted in alveoli. In some genera the 

 extremities of the jaws are edentulous, and seem to have been 

 sheathed with horn. 



The sclerotic has an osseous ring. 



The pectoral arch is extremely like that of a carinate bird, 

 but no clavicles have been discovered. The manus has four 

 digits, three of which are short and provided with claws, while 

 the fourth, enormously prolonged and clawless, appears to have 

 supported a "patagiurn," as in the Bats. 



The posterior limbs are comparatively small. 



The long bones and the vertebras appear to have contained 

 pneumatic cavities, as in many birds. 



5. The DINOSATJRIA. The bony exoskeleton is sometimes 

 more highly developed than in the Crocodilia, and sometimes 

 absent. The centra of the posterior dorsal vertebras are flat or 

 slightly concave at each end, and they have crocodilian trans- 

 verse processes and ribs. The centra of the anterior dorsal and 



