150 GENERAL ORNITHOLOGY. 



of them, articulate, is the pleurosteon (Gr. nKevpov, pleuron, a rib); in adult life this becomes 

 the costal process, so prominent in Passeres (fig. 58). The posterior lateral piece is the metosteon 

 (Gr. fifrd, meta, after). From the latter are derived the pair, or two pairs, of lateral processes 

 which the posterior border of the sternum has in so many birds. In fine, the extent of ossifica- 

 tion of the lophosteon and metostea, and the mode of their coosification, determines all those 

 various shapes of the posterior border of the sternum which, being commonly characteristic of 

 genera and higher groups, are described for purposes of classification. Thus, if the lophosteon 

 and the metostea are completely ossified and to the same extent behind, the posterior border of 

 the sternum will be transverse, and perfectly bony. Such a sternum is said to be entire. If the 

 lophosteon is longer than the lateral pieces, the sternum will have a central pointed or rounded 

 projection ; when such a formation is called the middle xiphoid process (Gr. ^icpos, xiphos, a 

 sword: etSoy, eirfos, form). The projection of the metostea, not infrequent, similarly gives 

 a pair of external lateral xiphoid processes. But such processes oftener result merely from de- 

 fects of coosification between the elements of the sternum. Thus, there is often a deep notch 

 in the posterior border of the sternum between the lophosteon and the metosteon of each side ; 

 the sternum is then said to be single-notched or single- emarginate (one pair of notches, one on 

 each side ; fig. 58). This conformation prevails throughout the great group Passeres, possibly 

 without exception ; it is therefore highly characteristic of that order, though a great many other 

 birds also have it. In the natural state, the notch is filled in with membrane. Such a notch 

 may also be converted into a "fontanelle" or fenestra (Lat. fenestra, a window), which is simply 

 a hole in the bone, the metostea having grown to the lophosteon at their extremities, but left an 

 opening between. Such a sternum is called fenestrate, more exactly uni-fenestrate (Lat. unus, 

 one; one window on each side). Now, the parts remaining as before, let either each half of 

 the lophosteon, or each metosteon, be notched or fenestrate ; obviously then, such a sternum is 

 double-notched or hi- fenestrate, having four notches, or holes, two on each side, — two notches, 

 or two holes ; or notched and fenestrate, having a notch and a hole on each side. The latter 

 is very frequent : when occurring, the hole is generally nearest the middle line, the notch ex- 

 terior. Irregularity of ossification, converting a hole into a notch, and conversely, may in any 

 case result in lack of symmetry; but this is a mere individual peculiarity. When there are 

 two notches on each side, as in fig. 56, the sternum has evidently a median and two lateral back- 

 ward extensions, which are then called respectively the middle, internal lateral, and external 

 lateral xiphoid processes. Notching of the lophosteon in the middle line, at least to any extent, 

 must be very rare, if indeed it ever occurs. The extreme case of emargination of the sternum is 

 afi"orded by the Gallince, and is highly characteristic of that group. Here the lophosteon is 

 extremely narrow, and fissured deeply away from the metostea, which latter are deeply forked ; 

 the arrangement giving rise to two very long slender lateral processes on each side (figs. 1 and 2, 

 p. 48). The sternum of the tinamou, a droma?ognathous bird, is still njore deeply emargi- 

 nated, but the extremely long and slender lateral processes, which enclose an oval contour, are 

 simple, not forked. 



In a very few birds there are centres of ossification additional to those above described. 

 In Turnix, there are said by Parker to be a pair of centres between the pleurostea, which he 

 names coracostea, because related to the part of the sternum with which the coracoids (see 

 p. 146) unite. The same authority describes for Dicholophus a posterior median cartilagi- 

 nous flap having a separate centre, named urosteon (Gr. ovpa, oura, tail). In various birds the 

 sternum is eked out in the middle line behind by cartilage which has no ossification. 



The sternum, especially of the higher birds, develops in the middle line in front a beak- 

 like process called the rostrum or manubrium (Lat. manubrium, a handle) ; its size and shape 

 vary ; it is well-marked in Passerine birds (fig. 58) ; and may be bifurcate at the end and run 

 down the front of the keel some way, as in the raven. The fore border of the sternum is 

 generally greatly convex from side to side, and then, in those birds which have prominent 



