SKELETON OF MAKSUPIALIA. 



S57 



227 



Pelvis and marsupial bones of the Koala. 



compression required to expel the uterine foetus. It is not in the 

 females of those animals which give birth to the smallest young 

 that we should expect to find 

 auxiliary parts for increasing 

 the power of the muscles en- 

 gaged in parturition. The 

 bones in question are, more- 

 over, equally developed in both 

 sexes : and they are so situated 

 and attached that they add to 

 the power of the muscles which 

 wind round them, and not of 

 those implanted in them. They 

 are not, however, merely sub- 

 servient to add force to the 

 action of the * cremasteres,' but 

 give origin to a great propor- 

 tion of the so-called ' pvrami- 

 dales.' 1 



The osteogenesis of the mar- 

 supial pelvis derives some ex- 

 trinsic interest from the not yet forgotten speculations which have 

 been broached regarding the homologies of the marsupial bones. 

 These have been conjectured to exist in many of the placental Mam- 

 malia, with a certain latitude of altered place and form, disguised, 

 e.g., as the bone of the penis in the Carnivora, or appearing as 

 the supplemental ossicles of the acetabulum, which exist in the 

 young of many of the Rodentia. In the os innominatum of the 

 immature Potoroo the curved prismatic ilium contributes to form, 

 by the outer part of its base, the upper or anterior third of the 

 acetabulum ; the rest of the circumference of this cavity is com- 

 pleted by the ischium and pubis, excepting a small part of the 

 under or mesial margin, which is formed by a distinct ossicle or 

 epiphysis of the ilium («, fig. 152), answering to that described 

 by Geoifroy St. Hilaire as the rudimental marsupial bone in the 

 Rabbit. Now here there is a coexisting marsupial bone : but 

 besides the five separate bones just mentioned, there is a sixth 

 distinct triangular ossicle, which is wedged into the posterior 

 interspace of the ischiopubic symphysis. The circumference of 

 the acetabulum is always interrupted by a deep notch opposite 

 the obturator foramen, which is traversed by a ligamentous bridge, 



' Sec the absti-act of a paper on the anatomy of tlie Dasyiirus, Proc. Zoo). Soc. 

 January, 1835. 



