THE DIVISIONS OF THE EEPTILIA. 195 



5 When an interclavicle exists it is confluent with the 

 clavicles. 



6. The manus does not contain more than three digits, and 

 not more than the two radial digits have claws. 



7. The ilia are greatly prolonged in front of the aceta- 

 bulum, the inner wall of which is membranous. The pubes 

 and ischia are directed backwards, more or less parallel 

 with one another, and the ischia never meet in a ventral 

 symphysis. 



8. The astragalus sends up a process on to the front face 

 of the tibia, and early ankyloses with the latter bone. In 

 this character. Birds differ from all existing Reptiles. The 

 foot contains not more than four digits. The first meta- 

 tarsal is, almost always, free, shorter than the rest, and 

 incomplete above. The other three are ankylosed together. 

 and with the distal tarsal bone, to form a tarso-metatarsus. 



Some of the extinct Dinosauria closely resembled birds 

 in the form of the tibia and astragalus, the immoveable 

 union of the two bones, and the reduction of the number 

 of the digits. 



9. Only one aortic arch, the right, is present. Only one 

 arterial trunk, the pulmonic, is given off from the right 

 ventricle. The arterial and venous currents communicate 

 only by the capillaries. 



10. The blood is hot. There are three semilianar valves 

 at the origins of the aortic and pulmonary trunks. In all 

 existing birds the extremities of the chief pulmonary 

 passages terminate in air-sacs. There is a rudiment of this 

 sti-ucture in the Chamseleons, and the extinct Pterodactyles 

 very probably possessed such sacs. 



11. The corpora higemina are thrown down to the sides 

 and base of the brain. 



The Septula. — This class is divisible, by well-defined 

 characters, into the following groups : — 



A. The dorsal vertebrae (which, like all the other vertebrae, are 

 devoid of transverse processes) are not moveable upon one another, 

 nor are the ribs moveable upon the vertebrae {Pleurospondylia). Most 



