434 AVES. 



other ventrally except in the ostricli in which there is a sym- 

 physis pubis, and in Rhea in which there is a symphysis ischii ; 

 but the symphysis ischii of Rhea is pecuHar in being dorsal to 

 the intestine and ventral to the kidneys ; it is associated with a 

 curious weakening of the postsacral part of the vertebral column 

 in the adult. All three bones enter into the acetabulum which 

 is perforated, and the pubis has in some birds {Apteryx) a for- 

 wardly directed preacetabular process (pectineal process). 

 This process appears to be more marked in embryos, in which it 

 was discovered by Miss Johnson * in the chick. In a few birds 

 the ilia are quite separate from the ischia except in the aceta- 

 bulum [Tinamus, Struthio, Apteryx, Hesperornis, etc.). The 

 short and powerful femur is directed obliquely horizontally for- 

 wards and concealed beneath the flesh and feathers of the abdo- 

 men, so that the knee-joint is not visible externally. The head 

 is rounded and stands out at right angles to the bone. A patella 

 is usually present. The cms, which is much longer, is chiefly 

 composed of the tibia, the fibula being reduced (especially dis- 

 tally) and represented by a styliform bone on the outer side of the 

 tibia. The proximal end of the tibia is expanded and has on its 

 anterior face a great ridge, the cnemial crest. There appears 

 to be no tarsus, but two rows of tarsal elements are present in 

 the embryo, of which the proximal row unites with the tibia and 

 the distal with the metatarsus, so that the bone which we have 

 called tibia is really the tibio-tarsus, and the ankle joint is inter- 

 tarsal. In the same way the metatarsus is in reality a tarso- 

 metatarsus. The tarso -metatarsus varies much in length and 

 is the cause of the differences in the length of the leg. It is 

 composed of the distal tarsalia and of three long metatarsal 

 bones of digits 2, 3, and 4, fused together. At its lower end it 

 discloses its composite nature by dividing into three j)i"ocesses 

 which are provided with articulating surfaces for the proximal 

 phalanges of the corresponding digits. When a fourth toe 

 (digit No. 1) is present, its metatarsus is distinct from the tarso- 

 metatarsus and has the form of a small bone on the inner side 

 of the metatarsus ; it carries the phalanges of the inner digit or 

 hallux. The usual phalangeal formula is 2, 3, 4, 5. No bird 

 has digit No. 5,| though a trace of its metatarsal is said to be 

 present in the embryo. 



* Q.J. M.S., 23, 1883, p. 399. 



t The fifth digit seen in some breeds of fowl is not the true fifth digit, 

 but an abnormality. 



