THE SKELETON IN MAMMALIA. THE STERNUM. 489 

 THE STERNUM 1 . 



In MONOTREMES and most MARSUPIALS the sternum does 

 not present any characters of special importance. The pre- 

 sternum is strongly keeled in Notoryctes. 



The sternum in EDENTATES is very variable : in the Sloths 

 it is very long, the mesosternum of Choloepus having twelve 

 segments. In the anteaters and armadillos the presternum 

 is broad and sometimes as in Priodon strongly keeled. In 

 Manis macrura the xiphisternum is drawn out into a pair of 

 cartilaginous processes about nine inches long. 



In the SIRENIA the sternum is simple and elongated, and 

 of fairly equal width throughout, in the adult it shows no sign 

 of segmentation. Its origin from the union of two lateral 

 portions can be well seen in Manatus. 



Two distinct types of sternum are met with in the CETACEA. 

 In the Odontoceti the sternum consists of a broad presternum 

 followed by three or four mesosternal segments, but with no 

 xiphisternum. Indications of the original median fissure can 

 be traced, and are very evident in Hyperoodon. In the 

 Mystacoceti, on the other hand, the sternum consists simply 

 of a broad flattened presternum which is sometimes more or 

 less heart-shaped, sometimes cross-shaped. Only a single pair 

 of ribs are united to it. 



The sternum in UNGULATA is generally long and narrow and 

 formed of six or generally seven segments. The presternum 

 is as a rule small and compressed, often much keeled, especially 

 in the horse and tapir. The segments of the mesosternum 

 gradually widen as followed back and the xiphisternum is 

 often terminated by a cartilaginous plate. 



In the RODENTIA the sternum is long and narrow and 

 generally has a large presternum, and a xiphisternum termi- 

 nated by a broad cartilaginous plate. 



1 See W. K. Parker, Monograph of the shoulder-girdle and sternum 

 of the Vertebrata, Ray Soc. 1868. 



