104 THE STERNUM. [CHAP. 



In the Armadillos (Dasypodidce) the presternum is broad, 

 and in Priodon gigas (Fig. 45, p. 114) strongly keeled. The 

 mesosternal segments, four to six in number, are broad above, 

 but very narrow below. They are connected by synovial 

 joints to each other, and to the strongly ossified sternal ribs, 

 which have broad, sub-bifid heads. The xiphisternum 

 expands posteriorly into a wide cartilaginous flap. 



In the Sloths the sternum is long and narrow. The Three- 

 toed species {Bradypus} have a rather broad presternum, but 

 with no prolongation in front of the attachment of the first 

 rib. This is followed by eight small mesosternal segments, 

 and a very small rounded xiphisternum. In the Two-toed 

 Sloths (Cholazpus) the presternum is narrow, slightly keeled, 

 and forms a considerable projection in front of the attach- 

 ment of the first rib. The mesosternum has twelve segments, 

 and the xiphisternum is rudimentary or absent. 



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, con- 

 nected 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. 44) has 

 a broad presternum (ps\ with a small partially-ossified 

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

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

 no xiphisternum. The Echidna agrees in all important 

 respects, but it has an ossified xiphisternum, which in E. 

 Bruijni consists of three metameric portions. 



