BIRDS. 147 



The rostrum (r.) is already evident, and the external xiphoid process (e. x.) is differentiated 

 from the middle part (m. x.) by a notch ; but there is no fissure botween the intermediate and 

 middle xiphoid in this species at any stage. The primordial division of the two halves of the 

 Sternum is shown behind ; and the xiphoid region, altogether, is small only half the length of 

 the Sternum, and very narrow. Seven pairs of sternal ribs have synovial joints with the Sternum, 

 and an eighth pair nearly reaches its edge. In Plate XVI, fig. 23, the position of the three clavicular 

 bones at the next stage, as seen from behind, is shown, magnified twenty diameters ; the clavicles 

 (cl.) are united by suture, and beneath them is the inter-clavicle (i. cl.) ; whilst the soft and very 

 transparent tissue, in which it is imbedded, is the feeble rudiment of a right and left pair of distal prse- 

 coracoids, a structure well developed in many Mammals. The most important change to be noticed 

 in this further stage, when the embryo is twice the size of the one last described, is the formation of a 

 delicate mass of fibre-cartilage, the " distal praa-coracoid" (d. p. c.), in the middle of which the inter- 

 clavicle, now very distinct, can be seen. In an embryo three or four days before hatching (see 

 Plate XVII, fig. 14, which is magnified three diameters), we find several things to be noticed; and 

 these are the advance of the ossifying shafts; the more perfect fission of the scapula (sc.) from the 

 coracoid (cr.) ; and especially the forward elongation of the head of the latter, on which the proximal 

 prse-coracoid (p. p. c.) is well seen, as a thick wedge of clear cartilage, as yet quite distinct from the 

 clavicle ; this latter part, however, has quite used up the meso-scapular segment, and the soft 

 tissue below has been ossified by the inter-clavicle (i. cl.). The whole form of the " furcula" is 

 more like what is seen in the adult ; and so is that of the true Shoulder-girdle. The rostrum of 

 the Sternum (r.) is now well developed ; but both it, the rest of the Sternum, the sternal ribs (s. r.), 

 and the costal appendages (a. p.), are all unossified, as is much of the vertebral ribs (v. r.), espe- 

 cially behind. The keel (k.) is now growing forwards under the rostrum, and the external xiphoid 

 process (e.x.) has attained its proper shape; whilst the post-costal part of the Sternum is rela- 

 tively larger, and the meso-xiphoid notch is filled up behind. The eighth sternal rib reaches the 

 Sternum in this instance ; then comes another which reaches nearly as far downwards, and after 

 it there is an abdominal rib (a. r.) which, however, has a distal vertebral portion ; this well illus- 

 trates the lateral deficiency of cartilaginous arches, and how they crop out below. The costal 

 appendages (a. p.) are curious rays, growing out of and segmenting themselves from the vertebral 

 ribs ; there are eight pairs of them, and they have very fanciful shapes. The bony structures of 

 the Shoulder and Sternum in the adult can now be understood (see ' Osteol. Catal. Hunt. Mus.' 

 vol. i, p. 223, No. 1164). 



The scapula is very long and narrow, and is blunt-pointed ; the coracoid is strongly hooked 

 in front, and the prae-coracoid segment, which articulates with the flattened end of the hook, is 

 thus carried far from the shaft. The meso-coracoid region is defined by an inferior groove, and 

 by the large coracoid foramen ; the epicoracoid region sends upwards a small sharp hook. The 

 pra3-coracoid segment is one with the furcular ramus (clavicle), and reaches to the most curved 

 part of the strongly bent bar ; this bar thickens where it curves inwards to meet its fellow. 

 Behind the junction of the two clavicles the inter- clavicle grows backwards as a notable semi-oval 

 plate. The Sternum is long-oblong very long indeed, and is only a little narrow behind the 

 last sternal rib ; this rib only marks the end of the first third of the Sternum. The rostrum is 

 large and well marked ; the keel runs far forwards ; the external xiphisternal process is separated 

 by a narrow long notch from the intermediate and middle part, which have no cleft between 

 them. The posterior margin of the Sternum, howeveV, is trilobate, showing its triple nature, 



