MAY, 1921. AMERICAN MARSUPIAL, C^NOLESTES OSGOOD. 87 



sternum and are separated at their extremities by the xiphisternum,where- 

 as in Perameles and most diprotodonts the condition is as in C&nolestes. 



The manubrium of the sternum is like that of most of the smaller 

 claviculate marsupials. It consists of an anterior prong which is some- 

 what deflected ventrad and slightly faceted dorso-cephalad for the 

 clavicular union. A small triangular lateral process projects from each 

 side of the base of this prong and the body of the bone extends thence 

 caudad as a subcylindrical rod with a slight keel on its ventral surface. 

 In a male specimen the manubrium is 7 mm. in length. 



The four segments of the mesosternum are of nearly uniform width 

 throughout without pronounced median constriction or lateral expansion 

 even in the fourth which is scarcely wider posteriorly than anteriorly. 

 The first is somewhat more rounded than the others which are slightly 

 flattened but with their ventral surfaces rather more convex than other- 

 wise. 



The xiphisternum is a long slender bony rod with slightly greater 

 width than thickness, but, in general, rounded in form especially proxi- 

 mally where it is somewhat expanded. Distally it has a more abrupt and 

 more flattened slight expansion where it joins the xiphoid cartilage, 

 which is relatively large and scutate. The xiphisternum is separated 

 from the last segment of the mesosternum dorsally by a broad epiphysial 

 cartilage and ventrally by the extremities of the seventh pair of ribs. 

 Hence, although the plane of the dorsal surface of the xiphisternum is 

 continuous with that of the mesosternum, its ventral plane is distinctly 

 higher. This epiphysial cartilage and this relation of the proximal end 

 of the xiphisternum, although apparently trivial, are nevertheless 

 characteristic of the diprotodonts as opposed to the polyprotodonts. 

 In all polyprotodonts examined, except Perameles, both surfaces of the 

 xiphisternum are practically continuous with the corresponding surfaces 

 of the mesosternum and the sixth or seventh ribs do not meet ventrad 

 of the base of the xiphisternum. In all diprotodonts, including Cazno- 

 lestes, the reverse is the case. Perameles, as judged by one specimen, 

 seems to occupy an intermediate position in this respect, for although 

 ribs meet on the end of the mesosternum, the epiphysial cartilage is so 

 reduced as to be scarcely noticeable in a dried specimen. 



PECTORAL GIRDLE. 



The clavicle (PI. XII, fig. 5) is well developed and stretches from the 

 sternum to the acromion in the manner usual among marsupials. Its 

 lateral or acromial end is compressed and curved upward. In the adult, 

 the union with the acromion is almost direct, the intervening cartilage 

 being scarcely evident. It is attached also to the coracoid process by a 



