326 THE PELVIC GIRDLE. [CH. xvir. 



category of sesamoid bones. They vary in size and shape 

 in different species. In Didelphys they are nearly as long 

 as the ilia, while in the Kangaroo they are scarcely half the 

 length of that bone. Though largely developed in the 

 Dasyures, in the allied genus Thyladnus they are repre- 

 sented only by small, unossified fibre-cartilages. 



These bones are commonly called " marsupial bones," 

 though they have no special function relating to the ventral 

 pouch of the female, being nearly equally developed in both 

 sexes, and also in those species in which the marsupium is 

 not present. 1 



In the MONOTREMATA the pelvis is short and broad. 

 The ilia are short, distinctly trihedral and everted above. 

 The ischia are large, and prolonged into a considerable 

 backward-directed tuberosity. The symphysis is long, and 

 formed about equally by pubes and ischium. The thyroid 

 foramen is round. The acetabulum is perforated in Echidna 

 as in Birds, but not in Ornithorhynchus. The pectineal 

 tubercle is greatly developed. There are large "marsupial ' : 

 bones in both genera. 



1 See O. Katz, " Zur Kenntniss der Bauchdecke und der mit ibr 

 verkniipften Organe bei den Beutelthieren. Zeitschr. f. wiss Zoolog." 

 36, 1882, p. 611. 



