256 EEPTILIA class in 



Class 4. AVES. Birds. 1 



Feathered, warm-blooded, oviparous vertebrates with chambers of the heart 

 completely separated. There is a single occipital condyle, the quadrate is free, and 

 the fore-limbs are modified into wings. Proximal tarsals fused with tibia to form a 

 tibio-tarsus ; metapodol bones fused with one another, and with the distal tarsals to 

 form a tarso-metatarsus. Fifth digit of the pes always absent. 



Of all classes of vertebrates, birds constitute the most homogeneous and 

 aarrowly circumscribed group, and exhibit everywhere the most remarkable 

 uniformity. Descended without question from reptiles, their affinities with 

 that class are so intimate that Huxley included them both under the common 

 designation of Sauropsida. This proposed merging of the two classes, however, 

 is hardly compatible with such trenchant distinguishing characters as the 

 epidermal covering of feathers, heterocoelous vertebrae, and warm - blooded 

 (homothermic) condition. 



The exosheleton of birds consists of feathers, the horny covering of the beak, 

 claws, and spurs, and certain corneous plates often found on the tarsus and 

 feet. Save in penguins the feathers do not cover the body uniformly, but 

 are arranged in certain definite tracts (pterylae), between which are bare 

 spaces (apteria). These apteria are best seen on the abdomen and on the 

 sides of the neck in many birds. Although struthious birds are also com- 

 monly said to be uniformly covered with feathers, Pycraft has shown the 

 presence of small apteria in most species. Under exceptionally favourable 

 conditions, as in the Lithographic Stone of Bavaria, and the Green Kiver 

 Eocene of Wyoming, imprints of feathers may be preserved in the rocks. 



The skeleton of birds is remarkable for its combination of compactness and 

 lightness, and for its pneumaticity, or permeation by air cavities. The walls 

 of the bones are very thin, but of dense texture, owing to their richness in 

 calcium phosphate. In young birds the cavities of the long bones are filled 

 with marrow, and this may persist in some bones throughout life, as in a 

 number of water birds ; on the other hand, the marrow may disappear and 

 the interstices become filled with air, as in the long bones of birds which 

 sail or soar, and many others ; finally, the long bones may be completely 

 filled with spongy bone or cancellar tissue, as is the case with penguins. 

 The skull, humerus, and femur may be said to be pneumatic in the majority 

 of birds. 



The vertebral column of birds is divisible into cervical, dorsal, sacral, and 



1 Literature : 



Beddard, F. E., The structure and classification of Birds. London, 1898. — Fiirbringer, M., 

 Untersuchungen zur Morphologie und Systematik der Vbgel. Amsterdam, 1888.— Huxley, T. H., 

 On the classification of Birds (Proc. Zool. Soc. London, p. 415), 1867. — Lydekker, R., Catalogue of 



il Birds in the British Museum. London, 1891. — Marsh, 0. C, Odontornithes : a monograph 

 of the extinct toothed Birds of North America. Washington, 1880.— Meyer, A. B., Abbilddungen 

 von Vogel-Skeleten. Dresden, 1879-90. — Menzbier, M. von, Vergleichende Osteologie der Pinguine 

 in Anwendung zur Haupteintheilung der Vbgel (Bull. Soc. Imp. Moscow), 1887.— Milne- Edwards, 

 .1.. Rrecherches anatomiques et pah'ontologiques pour servir a l'histoire des oiseaux fossiles de la 

 France. Paris, 1867-72.— Newton, A., Dictionary of Birds. London, 1893-96.— Pycraft, W. P., 

 ributions to the osteology of Birds (Proc. Zool. Soc. London), 1898 et seq. [An important series 

 of papers not yet (1901) completed.]— Selenka, E., and Gadow, H., Aves, in Bronn's Classen und 

 Ordnungen des Thierreichs, 1869-91.— Seeley, H. G„ On the British fossil Cretaceous Birds (Quar. 

 Journ. Geol. Sue. vol. XXXII.), 1876. 



