150 SHOULDER-GIRDLE AND BREAST-BONE. 



very square, the length being but little greater than the breadth. The figures showing the next 

 stage (4 6) are reduced one fourth below the natural size ; fig. 4 shows the Sternum from above, 

 fig. 5 from below, and fig. 6 the Shoulder-girdle. The osseous shafts of the scapula (sc.) and 

 coracoid (cr.) are considerably advanced, and the proximal prse-coracoid segment (p. cr.) is 

 very much larger, and has acquired a bony mass within ; a section of this mass is shown in fig 9, 

 magnified three diameters; c. is the outer cartilage, and en. o. the endosteal deposit. The 

 shape of the Sternum (figs. 4, 5) has become considerably changed; a small rostrum (r.) has 

 been developed from the top of the now far-projecting keel (k.), and the external xiphoid process 

 (e. x.) has grown backwards into a flat, beak-shaped flap, almost aborting the intermediate process 

 (i. x.) ; whilst the middle process (in. x.) is well defined, but shows the Reptilian character of the 

 Sternum by its ending so far in front of the outer part. The ear-like costal processes (c. p.) are 

 much more developed, and so are the costal condyles (c. c.) ; the fifth is aborted. The coracoid 

 grooves (cr. g.) now overlap each other, the right passes below the left, and the furcular angle 

 (cl. i. cl.) articulates with the projecting end of the sternal keel ; this crest only occupies the 

 anterior half of the Sternum. A large bilobate tract of bone has appeared in the body of the 

 Sternum, on each side ; this is the " pleurosteon ;" it is principally internal entirely at first, 

 but is reaching the surface in its mid-region. The pleurosteon is more developed above 

 (fig. 4) than below (fig. 5), where the right and left tract are still far apart. The keel (k.) 

 is the subject of another bony deposit the " lophosternum " (1. o.) ; this commenced in the 

 thickness of the cartilaginous ridge, near its anterior margin. Fig. 7 shows a section (three 

 diameters) taken in a vertically transverse direction through the coracoid grooves ; the bony 

 matter (p. o., 1. o.) is seen to be almost entirely within the cartilage ; the " centres " are far apart. 

 Fig. 8 shows a similar section taken in front of the last : there the deposits have begun to meet, 

 and the " lophosternum " (1. o.) has gained the outer surface. 



In the adult Cormorant all the parts become intensely ossified, and the prse-coracoid 

 completely coalesces with the clavicle; the Sternum is one dense bone, which, 'however, retains 

 some soft cartilage behind, both at the mid-line and at the end of the external processes. The 

 keel projects an inch in front of the coracoid grooves ; it dies out behind, the part behind the last 

 costal condyles being as much rounded as in the Ostrich's Sternum. One marked peculiarity in 

 the Shoulder-girdle of the Cormorant is the very great length of the scapula, coracoid, and furcular 

 rami, in proportion to that of the Sternum ; and another is the sudden angle at which the lower 

 part of each furcular ramus is bent upon itself at the lower third of the prse-coracoid ; it is less 

 than a right angle, and the space for the penniform " middle pectoral muscle," which is formed 

 by the thick prse-coracoid above, the coracoid behind, the clavicle in front, and the sternal keel 

 below, is widely oval. The line of segmentation between the head of the coracoid and the great 

 " prae-coracoid segment " has become a large, oval, gliding synovial joint. 



Example 2. Sulafusca, Viell. 



Figs. 10 12 show the Shoulder-girdle of a young Booby reduced to one fourth below the 

 natural size. The supra-scapula (s. sc.) is at this stage like what is persistent in the Reptilia, 

 namely, it is ossified by endostosis only ; but this was derived, evidently, from the scapular shaft 

 was not independent. The coracoid (cr.) is almost entirely ossified, and so also is the prae-cora- 

 coid (p. cr.) ; but this latter bone is still distinct from the clavicle (cl.) ; it projects considerably in 



