86 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



the absence of spinous processes on the proximal caudals is a more 

 frequent condition among Australian than American forms. With the 

 exception of Perameles, which has six caudal spines, and Phascolarctos 

 and Phascolomys, with practically complete sets, all the Australian 

 forms examined either have no caudal spines or have them few and 

 extremely rudimentary. In no form are they so completely absent as in 

 Canolestes. On the other hand, there are no less than four in Marmosa, 

 three in Peramys, and three to six in Didelphis. 1 Slight caudal spines 

 are reported in Notary ctes (Stirling 1891). 



The angle at which the sacrals are set between the ilia is less in 

 C&nolestes than in most of the polyprotodonts examined and, as in 

 Trichosurus and Macropus, the spinal axis is nearly parallel with the 

 longitudinal axis of the superior outline of the pelvis. Thus, if the 

 caudals are extended in the axis of the sacrals the transverse processes 

 of the fourth and fifth caudals lie below the ischial tuberosity instead 

 of well above it as in Didelphis, Dasyurus, Perameles, Myrmecobius, etc. 



RIBS AND STERNUM. 

 Plate XII. 



There are thirteen pairs of ribs as usual, seven pairs articulating 

 directly with the sternum and six pairs not reaching it. Nine pairs are 

 doubly articulated with the vertebrae by tubercles and capitulae. The 

 succeeding four pairs lack tubercles and arise from the bodies of the 

 vertebrae. The first rib is much heavier than the others and its costal 

 element is longer than the vertebral which is true of no other marsupials 

 examined except Trichosurus and Petaurus. Its costal element is fully 

 ossified, basally flattened and slightly bicipital near its union with the 

 lateral extension of the manubrium. The vertebral element is similarly 

 expanded at the head and narrowed toward its union with its fellow. 

 A strong first thoracic rib, it may be noted, is an edentate character. 

 The remaining ribs from the second to the thirteenth are very slender 

 and subcylindrical, especially the vertebral series. The sternal series 

 are slightly flattened, the seventh being rather noticeably wider than 

 the others. The sternal ribs from the first to the seventh show a greater 

 degree of ossification than usual in marsupials. The seventh pair of 

 costals articulate directly with the caudal end of the last segment of the 

 mesosternum and therefore lie ventrad of the xiphisternum and nearly 

 or quite touch each other at this point (PL XII, fig. 2). In Didelphis and 

 most other polyprotodonts these ribs only reach the sides of the meso- 



^Although six caudal spines are distinguishable in Didelphis virginiana, a speci- 

 men of D. paraguayensis from Ceara, Brazil, has only traces of three and the spine of 

 the second sacral also is undeveloped. 



