Zoological Society. 451 



testicle or mammary gland ; and the uses of these bones immediately 

 relate to those muscles. 



'* With reference to the interesting question — What is the homo- 

 logy or essential nature of the ossa marsupialia ? I have, on a pre- 

 vious occasion, discussed that problem before the Zoological Society, 

 and have not found reason to change the opinion I offered in 1 835 * ; 

 viz. that they belong to the category of the trochlear ossicles, com- 

 monly called sesamoid, and are developed in the tendon of the exter- 

 nal oblique which forms the mesial j'illar of the abdominal ring, as the 

 patella is developed in the rectus femor is, I'hey are not, however, 

 merely subservient to add force to the action of the ' cremas teres,' but 

 give origin to a great proportion of the so-called * pyramidales.' 



" The osteogenesis of the marsupial pelvis derives some extrinsic 

 interest from the not yet forgotten speculations which have been 

 broached regarding the analogies of the marsupial bones. These 

 have been conjectured to exist in m-any of the jDlacental Mammalia, 

 with a certain latitude of altered place and form, disguised, e. g. as the 

 bone oitlie penis in the Carnivcra, or appearing as the supplemental os- 

 sicles of the acetabulum, which exist in the young of many of the Ro- 

 dentia. 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 circumfe- 

 rence of this cavity is completed 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, analogous to that described by 

 Geoffroy St. Ililaire as the rudimental marsupial bone in the rabbit. 

 Now here there is a co-existing marsupial bone : but besides the five 

 separate bones just mentioned, there is a sixth distinct triangular os- 

 sicle, which is wedged into the posterior interspace of the ischio-pubic 

 symphysis. How easy to suggest that this single symmetrical bone 

 may be the representative of the os penis removed from the glans to 

 the root of the intromittent organ ! It is obviously a mere epiphysis 

 of the ischium. The circumference of the acetabulum is always in- 

 terrupted by a deep notch opposite the obturator-foramen, which is 

 traversed by a ligamentous bridge, and gives passage to the vessels 

 of the Harderian gland lodged in the wide and deep acetabular fossa. 



* See the abstract of a Paper on the analogy of the Dasyurus, Proc. Zool. 

 Soc, January 1835, in which the discussion of the question of the marsupial 

 bone is abridged in the fallowing words : " and Mr. Owen stated it to be his 

 opinion, that the marsupial bones are essentially ossifications of the tendons 

 of the external abdominal muscle which constitute the internal or mesial 

 pillars of the abdominal rings." The same hypothesis is again advanced in 

 the account of the anatomy of the Wombat. Proc. Zool. Soc. 1836, p. 49. 



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