4 



226 PROP. J. R. GREENE ON THE MUTUAL RELATIONS 



{Anterior limbs formed for flight. Bones light, hollow, with 

 air-cells Ord. Pteeosaubia. 

 Limbs ambulatory or natatory 4. 



{Body covered with a double shield, formed of both the exo- 

 skeleton and eudo-skeleton Ord. Chelonia. 

 Dermal armour, when present, formed wholly of the exo-skele- 

 ton and never presenting the form of a dou ble shield. . 5. 



_ J Sacral vertebrae never exceeding two 6. 



I Sacral vertebrae three or more 8. 



Eibs bifurcate. One external nostril. Anterior trunk-vertebraa 

 ■ftdtli two transverse processes. Skin protected by bony 

 scutes Ord. Crocodilia. 



Eibs with simple heads. Two external nostrils 7. 



r Vertebrae flat or somewhat biconcave. Limbs natatory. 

 I Ord. Sauboptervgii. 



7 < Vertebrae, in most, procoeliau, with a single transverse process 

 1 on each side. Limbs ambulatory .... Ord. Lacertilia, s. 

 ^ Sauria. 



f Sacral vertebrae three. Teeth with compressed crowns ; sharp, 

 t, I serrate ; in distinct sockets. Vertebrae biconcave. 

 I Ord. Thecodontia. 

 [ S^acral vertebrae four or more 9. 



{Vertebrae biconcave. Teeth absent or represented by a pair 

 of tusks Ord. Anomodontia. 

 Vertebrae flat or concavo-convex. Limbs ambulatory, long and 

 strong. Teeth numerous Ord. Dinosauria. 



Class AMPHIBIA. 



Occipital condyles two in mimler, or absent. No amnion or al- 

 lantois. Proximal ends of hyoidean and mandibular arches sus- 

 pended by a covimon pedicle. Gills and lungs. Heart, in most, 

 with two auricles. 



, f No occipital condyle. Notochord persistent 2. 



\ Two occipital condyles 3, 



r Limbs many -jointed, without digits. Opercular bones. 

 2 J . Ord. Lepidota. 



I Limbs with foirr digits. Post-orbitals and supra-temporals. 

 [ Head defended by ganoid plates .... Ord. Ganocephala. 



also may we distinguish between Serpents and those Saiirians in which hmbs 

 are absent. See, on these pomts, the work of Van der Hoeven, ah-eady re- 

 ferred to. 



