THE DEVELOPMENT OF THE MARSUPIAL SHOULDER GIRDLE. 765 



Assuming, then, that the main coracoid ossification, and that which forms the 

 coracoid process, is the homologue of the coracoid of the lower forms, in what light are 

 we to regard the secondary and more rudimentary ossification ? From its position on 

 the outer side of the glenoid cavity it might be taken to represent the precoracoid, but 

 such a view does not seem probable for a variety of reasons. Its development is 

 inconstant ; when present it always, I believe, forms part of the glenoid surface ; and it 

 is always late in developing. That it is merely an epiphysis with no special morpho- 

 logical significance seems to me to be the most natural interpretation of the structure. 

 In Echidna during the development of the coracoid two epiphyses form — one on the 

 posterior end, and a minute one, which however may not be constant, between the 

 coracoid and the scapula, at the posterior edge of the glenoid cavity. Though neither 

 of these epiphyses is homologous with the small ossification in the rabbit, the condition 

 in Echidna is interesting as showing that epiphyses may develop in connection with the 

 coracoid, in cases where it is practically impossible that the ossifications can have any 

 important morphological significance. 



In the mammalia during the development of almost all the cartilage bones epiphyses 

 are to be found. The reason for the formation of these epiphyses is probably not the same 

 in all cases, and yet, as a general rule, it may be stated that epiphyses form wherever, 

 for one reason or another, a considerable portion of cartilage remains, till comparatively 

 late in development, beyond the region of the central ossification. In the case of the 

 long bones the ends probably remain cartilaginous for a considerable time, to afford a 

 greater elasticity to the skeleton during the development of the bone. But in a number 

 of other cases, epiphyses seem to be formed to accomplish the ossification of tracts which 

 from some peculiarity of their structure are not readily ossified from the main centre. 

 Thus almost all bony processes are provided with epiphyses. As instances of this sort 

 in the human subject we may take the epiphyses of the tubercle of the rib, of the 

 anterior inferior spine of the ilium, or of the acromion. Of a similar nature would 

 seem to be the epiphysis on the outer side of the glenoid cavity in the rabbit and many 

 other of the higher mammals. The reason for its development may have been as 

 follows :- — The coracoid process has apparently been preserved in the higher mammals, 

 not to increase the glenoid surface, but because it gives an attachment to the biceps 

 and coraco-brachialis muscles, which is specially advantageous in that these muscles are 

 kept out clear from the working of the joint, and from pressing on the subscapularis. 

 The pull of these muscles, in course of time, has brought the centre of ossification of 

 the coracoid into the line of strain, as is well seen in the coracoid of, say, the young 

 baboon ; and the centre of ossification being thus brought somewhat back from the 

 glenoid cavity, a portion of cartilage is left on the anterior wall of the cavity to be 

 secondarily ossified as an epiphysis. 



We may next consider the significance of the anterior of the two coracoidal elements- 

 in the monotremes. This element which in the monotremes has usually been referred 

 to as the ' epicoracoid,' is admitted by practically every one to be the homologue of the 



VOL. XXXIX. PART III. (NO. 29). 6 C 



