144 
GENERAL OBNITHOLOGY. 
of them, articulate, is pleurosteon (Gr. liKevpov, ^ileuron^ a rib); in adult life this becomes 
the costal process, so prominent in Passeres (fig. 58). The posterior lateral piece is the metosteon 
(Gr. fxerd, 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. ^icposj xiplios, a 
sword: et8o9, m?os, 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 metostet)n 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" ov 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 
doiible-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 
afforded 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 dromseognathous bird, is still more 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 M^hich 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. maniibriiim, 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 
