80 FRANK BLAIR HANSON 



marsupial and most other mammals, all trace of this sterno- 

 coracoidal piece is lacking in the adult, it persists in the rodents, 

 and also in the narwhal (Monodon monocerus), and the black- 

 fish (Globicephalus melas), and possibly other animals. In 

 other words, in the rodents there is at first a complete coracoid 

 reaching from the scapula to the sternum ; later, degeneration of 

 the coracoid sets in at its middle portion, and working toward 

 each end, stops just in time to leave the coracoid process attached 

 to the scapula and the epicoracoid to the sternum. 



From the foregoing it is apparent that Trichosurus passes 

 through a stage in its development, as regards girdles and sternum, 

 which is directly comparable with that of the adult condition in 

 the monotremes. 



In a series of later papers ('98, '99 '02, '08) Broom studied a num- 

 ber of marsupials, both Polyprotodonts and Diprotodonts, and 

 found that conditions throughout the Marsupialia were as described 

 for Trichosurus. In a mammary fetus of the common phalanger, 

 14 mm. long, the well-developed coracoid articulates with the 

 sternum almost exactly as in the adult monotremes. In the 

 earliest stage of Dasyurus (fig. 39) studied by Broom, the cora- 

 coid is still large and reaches nearly to the sternum. It would 

 seem that here absorption had commenced at the medial end 

 rather than in the middle of the cartilage. Figure 48 (petrogale) 

 shows a complete, unjointed arch similar to that of Trichosurus. 

 Pseudochiurus and permales were studied in various stages and 

 agree in general with other marsupials, so that this is obviously 

 a normal and constant phenomenon of the marsupial embryonic 

 girdle, as it is of the adult girdle in the monotremes. 



Prior to Broom's work, it was difficult, practically impossible 

 in fact, to pass from the monotreme shoulder-girdle to that of 

 the marsupial, albeit the relationship of monotreme girdle to that 

 of the reptile was apparent. Since Broom's discovery, however, 

 of the complete girdle in the early marsupial, and also in the rat 

 by Paterson and the mouse and man by the present author, it 

 is clearly seen that the girdles and sternum of the higher mammals 

 are directly comparable to that of the marsupial fetus, and this 

 in turn to the girdle of monotremes, which are unquestioned in 



