THE DIVISIONS OF THE REPTILIJL 16U 



contains not more than four digits. The first metatarsal is, 

 almost always, free, shorter than the rest, and incomplete 

 above. The other three are anchylosed together, and with the 

 distal tarsal bone, to form a tarso-metatarsus. 



Some of the extinct JDlnosauria closely resembled birds 

 in the form of the tibia and astragalus, the immovable 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 ven- 

 tricle. Tlie arterial and venous currents communicate only by 

 the capillaries. 



10. The blood is hot. There are three semilunar valves at 

 the origins of the aortic and pulmonary trunks. In all exist- 

 ing birds the extremities of the chief pulmonary passages 

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

 in the Cham^eleons, and the extinct Pterodactyles verj'- prob- 

 ably possessed such sacs. 



11. The corpora higemina are thrown down to the sides 

 and base of the brain. 



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

 acters, into the following groups : 



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

 transverse processes) are not movable upon one another, nor are the ribs 

 movable upon the vertebrae {Pleurospomlylia). Most of the dorsal vertebrae 

 and ribs are restrained from motion by the union of superficial bony plates, 

 into which they pass, to form a carapace. 



Dermal bones, usually nine in number, one of which is median and sym- 

 metrical, and the others lateral and paired, are developed in the ventral walls 

 of the thorax and abdomen, forming di plastron. 



I. — Chelonia. 



B. The dorsal vertebrae (which have either complete, or rudimentary, 

 transverse processes) are movable upon one another, and the ribs upon 

 them. There is no plastron, 



a. The dorsal vertebrae have transverse processes, which are either 

 entire, or only very imperfectly divided into terminal facets {M-pe- 

 tospondijlid). 

 a. The transverse processes are long ; the limbs well developed, with 

 the digits united by the integument into a paddle ; the sternum and 

 sternal ribs are absent or rudimentary. 



II. — Plesiosauria. 



h. The transverse processes are short, and sometimes rudimentary; 

 the limbs present or absent; when they are fully developed, the 

 digits are free, and there is a well-developed sternum with sternal 

 ribs. 



8 



