GENERAL STRUCTURE. 12 I 3 



by transverse processes placed on the arch. The anchylosed series 

 of caudals, which are sometimes termed tiro-sacra/, are in some in- 

 stances, as in Archaopteryx and Rhea, followed by a considerable 

 number of free vertebras, but more usually by only a few, succeeded 

 by a triangular terminal bone, known as the py go style (fig. 1106), 

 which carries the tail feathers and glands, and represents several 

 coalesced vertebrae. In no known Birds are ossified intercentral 

 elements developed, with the exception of the inferior bar of the 

 atlas vertebra. Nearly all the dorsal ribs 

 develop tubercular and capitular pro- 

 cesses, and some have also uncinate pro- 

 cesses (fig. 1 106, up). The sternum has 

 a pair of grooves superiorly for the re- 

 ception of the coracoids ; and in the 

 Ratitae (fig. 1105) it is rhomboidal and 

 convex, without trace of a median keel, 

 its development taking place from two 

 lateral centres. In the majority of Cari- 

 natae the sternum (fig. n 06, st) is, how- 

 ever, elongated, and has a strong median Fig. no 5 .— Stemai region of th< 

 keel for the attachment of the pectoral sSpuL^o > ric ) oid. Reduced ' *' 

 muscles. In this type two membranous 



vacuities frequently exist in the posterior portion, which in the dry 

 skeleton form holes or notches, separated by bony processes, which 

 represent divisions of the Mammalian xiphisternum. In many 

 Carinatae, and especially the Passerines, there are also developed a 

 median manubrium sterni, and lateral costal processes for the attach- 

 ment of the ribs. The coracoid (fig. 1 106, c) in the Carinatae is an 

 elongated bone more like that of Crocodiles than that of Dinosaurs ; 

 it has no fontanelle, and articulates at an acute angle with the scap- 

 ula, from which it usually remains distinct. It takes an equal share 

 with the latter in the formation of the glenoid cavity for the head of 

 the humerus, and at its distal end may overlap its fellow. In the 

 Ratitae the coracoid (fig. 1 1 1 1 ) is, however, generally shorter and 

 more Dinosaurian-like, and may have a fontanelle, while its long 

 axis is either coincident or parallel with that of the adjacent portion 

 of the scapula, with which it is invariably anchylosed in the adult. 

 The scapula in Carinate Birds (fig. 1106, s) consists of a thin and 

 narrow plate of bone, often extending backwards to a considerable 

 length, and without any suprascapula. Occasionally an additional 

 bony rod is developed on the outer side of the scapula. The 

 glenoidal ends of both the scapula and coracoid are divided into a 

 glenoidal and a clavicular process in this order. In the same order 

 the clavicle is nearly always well developed, and fuses with its fellow 

 to form the U-shaped furcula (fig. 1 1 06, fu); but in the Ratitae 



