756 DR R. BROOM ON 



limit of the degeneration was determined by the muscular attachment. The clavicle 

 is very well ossified, but there is still no cartilage in connection with either end. The 

 omo-sternum is of smaller proportional size than in the earlier stages. At this stage 

 the whole shoulder girdle comes more to the front, so that the clavicles, the sternum, 

 and the coracoid processes lie practically all in one plane. 



In a Trichosurus embryo of 37 mm. greatest length the whole shoulder girdle differs 

 but little from that of the adult. The scapula is directed upwards and backwards. The 

 spine is very large, and divides the outer surface into two deep fossae. The acromion 

 resembles that of the adult in shape and proportions. The coracoid process is quite 

 small, being absolutely scarcely larger than in the 23 mm. foetus, and is far removed 

 from the sternum. The clavicle is directed from its inner attachment outwards, for- 

 wards, and slightly downwards. At this stage it shows a distinctly new feature — in 

 that cartilage is now found at both the outer and inner ends. This cartilage is evidently 

 a secondary development of the large cells which, in the earlier stages, tip the bone. It 

 would seem as though the cells at the ends of the bone develop more rapidly than the 

 rate at which the bone encroaches on them, and that hence the more peripheral cells 

 become chondrified. 



Parker (4) describes the shoulder girdle of a young specimen measuring 8 inches from 

 snout to root of tail. It differs but little from that of the earlier stages. Both ends 

 of the clavicle are tipped with cartilage — that of the outer end being called the ' meso- 

 scapular segment'; the inner, the ' unossified part of the prsecoracoid.' The coracoid 

 is small and ossified by a single endosteal centre. 



The latest stage of the developing girdle in my possession is that of a young one 

 two-thirds grown. In it the coracoid only shows one centre of ossification. Whether any 

 further centre of ossification may be found as the animal approaches its full size 1 am 

 unable to say, but if a second centre exists it must be very small. 



The Shoulder Girdle in a 16 mm. Mammary Fcetus of Pseudochirus percgrinus 



(figs. 6 and 12). 



This embryo of the Ring-tailed Phalanger is in a stage of development about equal 

 to that of the 17 mm. Trichosurus. The shoulder girdles of the two genera, however, 

 present a number of very dissimilar features. 



The scapula is almost quite flat. The acromion springs from the anterior horder 

 a little above the glenoid cavity. It passes outwards and downwards as a thick, rounded 

 process ; then, becoming flattened antero-posteriorly, it passes downwards and inwards 

 as a broad, somewhat flattened plate to meet the outer end of the clavicle. The glenoid 

 cavity looks almost directly downwards. 



The coracoid probably forms the inner and anterior third of the glenoid cavity. 

 The anterior part of the coracoid is an irregular cuboid structure projecting inwards 

 from the anterior border of the glenoid cavity, and giving attachment to the long head 



