262 



AYES class iv 



The femur in birds is short and stout, with the neck and head extending 

 inwards into the acetabulum at right angles to the shaft. The latter is 

 directed forwards and slightly downwards during life, and is concealed beneath 

 the flesh and feathers of the abdomen in such a way that the knee-joint is not 

 visible externally. Its distal condyles are large and antero - posteriorly 

 elongated, and a patella is present in most cases. The tibia is a stout bone 

 considerably longer than the femur. Its expanded proximal end exhibits a 

 procnemial crest, which may extend above the knee-joint, and the distal 

 extremity has a trochlea- like surface, transversely elongated, and slightly 

 hollowed in the middle. In young Eatites, and in the embryos of Carinate 

 birds, the presence of a suture near the distal end of this bone indicates that 

 an astragalus and calcaneum are fused with it, thus forming a tibio-tarsus. 

 In the Moas the united astragalus and calcaneum remain for some time quite 



free from the tibia. 



The fibula in birds is a degenerate bone, and best developed in the Moas, 

 where, although short, it is stout and free. In most birds it is more or less 

 fused with the tibia, and is longest in the penguins, the fish-hawk, and some 

 owls, where it reaches almost to the ankle-joint. 



The distal row of tarsals fuses with the coossified metatarsals to form a 

 tarso-metatarsus (Fig. 366). This bone is peculiarly characteristic of birds, 

 and its variations, together with those of the tibia, cause the differences in the 

 length of the leg among different forms, which are sometimes enormous. The 

 fifth metatarsal is never developed, and the first, when present, is always 

 rudimentary, being attached to the inner side of the tarso-metatarsus by 

 ligament, or more rarely by suture. Metatarsals Nos. n to IV are imper- 

 fectly united in Archaeopteryx, and completely separated in the embryos of 

 modern birds ; but in the adult condition they are always fused into a single 

 bone, although the three components are plainly distinguishable in penguins. 

 This metapodal element terminates distally in three pulley-like surfaces for 

 the articulation of the phalangeals. Generally the median condyle is thrust 

 forward in advance of the other two, and the modifications of this region 

 afford important taxonomic characters. There is remarkable constancy in the 

 number of phalanges present in the toes of birds, the formula being 2, 3, 4, 5 

 in almost all cases where the full complement of digits is present. The 

 hallux, however, is frequently wanting, and in the ostrich only digits Nos. ill 

 and iy are developed. An apparent exception to the usual formula occurs 

 among some swifts and goatsuckers, where owing to fusion of some of the 

 phalanges the formula is 2, 3, 3, 3. 



Fossil egg-shells or casts of the same have been obtained from the Cretace- 

 ous and various Tertiary horizons, but are naturally much less common than 

 bones of the skeleton. They belong for the most part to cursorial or wading 

 bird.-. The largest known eggs are those of Aejnjomis, from the superficial 

 deposits of Madagascar, which have a capacity of about eight liters. Foetal 

 bones are occasionally found in Moa eggs from New Zealand. Certain three- 

 toed footprints occurring in the Trias of the Connecticut Valley were formerly 

 ascribed to an avian origin, but are doubtless referable for the most part to 

 bipedal Dinosaurs. The presumable kinship between birds and Dinosaurs has 

 already been discussed under the head of the latter group 



In comparison to the large number of recent species, of which over 12,000 

 have been described, the 400 or 500 known fossil forms yield but an in- 



