THE HUMAN STERNUM 53 



E. Mammalia 



The mammalian sternum is usually divisible into the three parts, 

 presternum, mesosternum, and metasternum ; but there are considerable 

 variations in the form and disposition of the three elements, to a large 

 extent dependent upon and correlated with the arrangement of the shoulder- 

 girdle and ribs. 



Omithodelphia. At the outset it is to be noted that Ornithorhynchus 

 and Echidna present a sharp contrast to the rest of mammals in the possession 

 of a shoulder-girdle, of a complex, reptilian type, associated with a mammalian 

 form of sternum. For several reasons I propose first to describe and contrast 

 these two forms separately {Echidna (PI. X, Fig. 68), from a specimen in the 

 Zoological Museum of the University of Liverpool ; Ornithorhynchus 

 (PL X, Fig. 69), also from the University Museum). From a comparison 

 of Parker's figure" with the Liverpool specimens, I conclude that his 

 example of Echidna was immature and imperfect. 



The shoulder-girdles are very similar in Ornithorhynchus and Echidna. 

 The coracoids laterally and the interclavicular portion of the clavicles 

 in the median line articulate with the anterior margin of the presternum. 

 The presternum is also much alike In the two cases ; in all cases it is 

 separate from the mesosternum. The mesosternum in two examples oi 

 Echidna consists of two pieces. In Ornithorhynchus in one instance it is 

 composed of two pieces, in another of three. The metasternum is an 

 oval bone imbedded in cartilage in Echidna. It is absent in both examples 

 of Ornithorhynchus. 



One would direct particular attention to the mode of attachment of the 

 ribs. In Echidna the presternum carries the first two ribs. In Ornithorhyn- 

 chus the first rib articulates with the presternum ; the second with the 

 interval between the presternum and mesosternum. In both Echidna 

 and Ornithorhynchus the first piece of the mesosternum carries the third 

 pair of ribs. The fourth and fifth pairs of ribs articulate with the second 

 piece of the mesosternum (or, in one example of Ornithorhynchus, with the 

 second and third pieces respectively). The sixth pair In Echidna articulates 

 between mesosternum and metasternum ; in Ornithorhynchus, with the 

 lower end of the mesosternum. One noticeable feature of these sterna is 



