230 AVES. 



cipally inhabit sluggish rivers and ponds. Cistudo europeea Schneid. = lutaria 

 Gesn., the common Tortoise of South Europe and East Germany ; Emys caspica, 

 on the Caspian Sea, in Dalmatia and Greece ; Clielydra serpentina L., with very 

 sharp jaws, in North America. 



Fam. Chersidae. Land Tortoises. With high, arched, ossified carapace ; 

 head and feet retractile. The digits are immovably connected as far as the 

 nails to thick club-feet, with indurated soles. They live in damp and shady 

 localities in warm and hot climates, and feed on plants. Testudo graca L., 

 nemoralis Aldr., = marginata, South Italy ; T. tabulata Daud., in America. 



CHAPTER VIII. 

 Class IV. AVES * BIRDS. 



Warm-hooded oviparous animals, covered with feathers. The cham- 

 bers of the heart are completely separated. The right aortic arch 

 persists. There is a single occipital condyle, and the anterior limbs are 

 transformed into wings. 



As opposed to the poikilothermic Vertebrates (i.e., Vertebrates 

 whose temperature varies with that of the external medium) the 

 blood of Aves and Mammalia possesses a high temperature, which 

 remains tolerably constant in spite of the changing temperature of 

 the external medium. This maintenance of a constant temperature 

 demands above everything a great energy of metabolism. The sur- 

 face of all the vegetative organs, especially of the lungs, kidneys, and 

 alimentary canal, has a relatively greater extension in the warm- 

 blooded than in the cold-blooded animals. The operations of diges- 

 tion, preparation of blood, circulation and respiration are carried on 

 with much greater energy. With the need of a richer nourishment, 

 the processes of vegetative life take a disproportionately more rapid 

 course, and as the high and uniform temperature of the blood is a 



* Joh. A. Naumann, " Naturgeschichte der Vogel Deutschlands," 13 Bde., 

 Stuttgart, 1846-1860. 



C. Naumannia, " Archiv fiir Ornithologie." Herausgeeeben von Ed. Baldamus 

 Kothen, 1849. 



" Journal fur Ornithologie," Herausgegeben von J. Cabanis. Cassel, 1853-1874. 



"The Ibis," 1859. 



Tiedemann, " Anatomic und Naturgeschichte der Vogel," Heidelberg, 1810- 

 1814. 



C. E. v. Baer, " Entwickelungsgeschichte der Thiere," I. und II., 1828-18S7. 



Remak, " Untersuch. iiber die Entwick. der Wirbelthiere," Berlin, 1850-55 



Huxley, " On the Classification of Birds," Proceed. Zool. Soc., 1867. 



Gray and Mitchell. " The Genera of Birds," London, 1841-49. 



C. Sundevall, " Tentamen." Stockholm, 1872-73. 



