86 THE STERNUM. [chap. vii. 



In the Marsupialia the sternum presents no especial 

 aberrant characteristics. The presternum is rather broad at 

 the point of attachment of the first pair of ribs. Its anterior 

 extremity often does not ossify. There are usually four 

 quite distinct, elongated segments to the mesosternum, 

 connected to one another by fibrous tissue, and sometimes 

 completed at each end by epiphyses. The xiphisternum 

 has an elongated, narrow, ossified portion, and terminates 

 in a laterally expanded cartilage, which may contain one 

 or two endosteal bony patches. 



In the Monotremata the Ornithorhynchus (Fig. 43) has 

 a broad presternum (ps), with a small partially ossified 

 pro-osteon (po) in front of it ; three keeled mesosternal 

 segments (ms), which commence to ossify in pairs, and 

 no xiphisternum. The Echidna agrees in all important 

 respects, but it has an ossified xiphisternum. 



The T-shaped bone, interclavicle or episternum {ic) in front 

 of the presternum, which connects it with the clavicle, and 

 which appears to have no homologue among the other 

 Mammalia, belongs more properly to the shoulder-girdle 

 than to the sternal apparatus. 



