THE DIVISIONS OF THE REI'TILiA., 100 



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

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

 above. The other three are anohj'losed 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 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 

 Rrterial trunk, the pulmonic, is given off from the right ven- 

 tricle. The 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 Chamseleons, and the extinct Pterodactyles very prob- 

 ably possessed such sxcs. 



11. The corpora bigemina 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 vertebrse (which, like all the other vertebra, are devoid of 

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

 movable upon the vertebrte {Pleurospondi/lia). 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 a plastron. 



I. — Chelonia. 



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

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

 Ihem. There is no plastron. 



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

 entire, or only very imperfectly divided into terminal facets (Erpe- 

 to^pondt/lia). 

 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 



