INTRODUCTIOX. 13 



one, — the finrrors wbic-li arc lost are the fourth and fiftli. AVhcn two-jointed, the 

 pollex, or first tiiigir, usually carries a ]iointed claw, and so may also the second one, 

 if three-jointed. The third finger only supports a claw in Arc/ueopteri/x (three- 

 jointed). A reduction in the number of phalanges is noticeable as correlated with 

 specialization in other directions, hence being found commonly among the so-called 

 ' liigher ' birds. 



When the wings are closed, the bones are usually folded up in such a way that the 

 hand forms a sharp angle with and underneath the forearm, so that the tips of the 

 fingers ]ioiiit backward; this is not the case with the Struthionine birds, in which 

 the hand is directed forward. In the penguins the joints are nearly inflexible, and 

 the hand is directed downward nearly in the ])roIongation of the a.\is of the forearm. 



Reserving the account of the different structures of the pelvis to the remarks 

 heading those groujis in which it is of special interest, we would here only call atten- 

 tion to Professor Marsh's discovery of the separate condition of the pelvic elements in 

 Archcioj)tert/.r, in contradistinction to other birds in which they are anchyloscd 

 together in the adult state, ami to the loose way in which the pelvic bones are ti.\ed 

 to the vertebral column in the penguins. 



In all birds the thigh bone is shorter than the tibia, a rel.-ition nearly unkixjwn 

 among the re])tiles; the same proportion is found, however, in the very bird-like, but 

 still indisputably Dinosaurian reptile, Composoijnathus, from the Jurassic formation, 

 and in the Pterodactyls. The femoro-caudal muscle, which, for example, produces 

 the curious sideways movement of the duck's tail, is in .some birds inserted upon a tro- 

 chanter of the femur, which, according to Professor Dollo, is the homologue of the 

 third, or, as he proposes to call it, the fourth trochanter of the IijKanodon. The tibia 

 is sometimes provided with a large cnemial process in front of the knee, as in the loons 

 and grebes and the cxt'mri J/eKperoi-nis and C/ieinionu's ; the fibula is usually more 

 or less rudimentary; as a peculiarity of that part in Archtropteryx may be mentiimetl 

 that its distal end was jilaced in front of the tibia, as in Iguanodon, a position un- 

 known in other birds. The ])art following is the tarsus, but the collection of small 

 bones designated by that teriTi do not apjiear in the adult bird, for one of them, at least, 

 — viz., the astragalus, — anchyloses early to the lower end of the tibia, while the others, 

 in the same manner, are fused together with the metatarsals. The joint between the 

 leg and the foot, therefore, is no true ankle-joint, but mesotarsal (inter-tarsal) articn- 

 lation. This is a reptilian feature, and the recognition of the true nature of this joint 

 is of considerable importance. It should also be borne in mind that the bone which 

 in topogra])liical ornithology is termed 'tarsus' consists chiefly of the three meta- 

 tarsals fused together, and shouM consequently be styled metatai-sus. In the embry- 

 onic bird these three elements are separate, ami in the penguins they remain distinct, 

 as only the ends grow together. The condition of the metatarsus in Arclmopteryx 

 may have been similar, judging from the two dee]) grooves on the anterior surface of 

 it. The metatarsal of the first toe (hallux) is very small, and usually free. The toes 

 are in the great majority four; viz., the first, second, third, and fourth, the fifth being 

 always absent both in recent and in fossil binls. The first is often wanting, but in many 

 cases where it is not seen outside the skin it m.iy be found underneath it. The 

 second is rudimentary in a few genera of kingfishers, and the fourth in Cholitrnis. 

 Rarely the number of toes is reduced to two (the ostrich), the first and second being 

 atrophied. The normal number of jrtialanges are two, three, four, and five in the first, 

 second, third, and fourth toe respectively, and the inner plialanx is the longer one, the 



