264 A HISTORY OF BIRDS 



adult, and these may be reduced to two, as in the Ostrich. 

 The tarsal or ankle bones also pass during their development 

 through a stage reminiscent of an earlier condition of things. 

 Here the proximal row is represented by a cylindrical mass of 

 cartilage supporting a vertical shaft which is closely applied to 

 the lower end of the tibia. In the old Dinosaurs this mass 

 remained distinct throughout life, in the bird it fuses soon after 

 hatching with the shaft of the tibia. A distal row of carpal 

 bones can be plainly made out in the embryo, but in the chick 

 these fuse to form a tabular mass closely applied to the bases 

 of the tarsals which form three distinct bones. Later these 

 become welded to form a " cannon " bone, and with the base 

 thereof the tabular mass of fused tarsals becomes indistinguish- 

 ably blended . Thus the ankle joint is " intertarsal," as in reptiles. 

 In the skull much evidence of a similar kind is to be met 

 with. Basipterygoid processes, or buttresses of bone project- 

 ing from the base of the skull for the support of the pterygoid 

 bones, are invariably present among reptiles. In birds they 

 are large only in the Ostrich tribe. Among the more modern 

 types they are small, or absent altogether in adults, yet where 

 not even a vestige in the adult remains traces will often be 

 found in the embryo. Again, though the earliest birds, like 

 reptiles, possessed teeth, these are conspicuous by their absence 

 in modern birds, though traces of these structures are said to 

 be met with in the beak of the embryo Parrot, and tooth-like 

 denticles are met with in the jaws of young Tinamous. On 

 the other hand, recently acquired characters, like the long 

 beak of the Avocet, the flat beak of the Spoon-bill, the crossed 

 beak of the Cross-bill, the serrated beak of the Darter and 

 Gannet, are sought for in vain in the embryo. The long, 

 slender, rod-like beak of the Humming-bird is a quite late 

 development. In the nestling the beak is short like that of a 

 Swift. Similarly, in the Cormorant, Gannet, Pelican and Penguin 

 the external nares have become suppressed, the fossa, therefor, 

 in the skull having become filled up by bone in all save the 

 Penguin ; but in the embryo and very young nestling this fossa 

 is as well developed as in normal birds. In Sphenisus only, 

 among the Penguins strangely enough, is there any tendency to 

 similarly close up this fossa, though the aperture in the horny 

 sheath is completely closed in all the Penguins. 



