THE ORNITHODELPHIA AND DIDELPHIA. 89 



urethral canal, but it is open and interrupted at the root of that 

 organ. Tn both sexes, the ureters pour the renal secretion, not 

 into the bladder, which is connected with the upper extremity 

 of the cloaca, but into the latter cavity itself. 



In the brain, the corpus eallosum is inconspicuous, though 

 the question how far it can properly be said to be absent 

 requires much more thorough investigation than it has yet 

 received.* We are but very imperfectly acquainted with the 

 reproductive processes of these animals, but it is asserted that 

 the young are devoid of a placenta. The mammary gland has 

 no nipple. 



Like the Ornithodelpliia, the division Didelphia contains 

 but a single order, the Marsupialia, the great majority of which, 

 like the Oriiitlwdelphia, inhabit Australia. They almost all 

 have the angle of the lower jaw inflected, and all possess true 

 teeth. The coracoid is, as in the higher Mammals, anchylosed 

 with the scapula, and is not articulated with the sternum. All 

 have the so-called " marsupial ' bones or cartilages — ossifica- 

 tions, or chondrifications, of the internal tendon of the external 

 oblique muscle of the abdomen — and the females of almost all 

 possess a fold of the skin of the abdomen above the pubis, con- 

 stituting a " marsupium" or pouch, within which the young are 

 nourished and protected in their early, helpless condition. 



The oviducts open into vaginse, which are more or less com- 

 pletely divided into two separate passages. The testes of the 



* For a number of years I have entertained the gravest doubts respecting the 

 accuracy of the doctrine put forth now nearly thirty years ago by Professor Owen, 

 and almost universally received, that the corpus eallosum is absent in Monotremes 

 and Marsupials, and at one time I began to collect materials for the thorough in- 

 vestigation of the question ; but other occupations intervened, and the plan was 

 never carried out. Nevertheless, I have always expressed myself cautiously 011 

 this subject, and, as the text shows, I was particularly guarded when delivering the 

 present lecture. At that time, in fact, I was well aware that my friend Mr. Flower 

 had commenced a series of inquiries into the question, and such results as he had 

 then obtained tended greatly to the increase of my scepticism. Mr. Flower has 

 since been good enough to go carefully with me over the large series of drawings 

 and preparations which he has made ; and I am prepared 1o express my entire 

 concurrence in his conclusion that the corpus eallosum exists, distinctly developed, 

 though not so well as in monodelphous, or placental, Mammals, in both the Bidvl- 

 phia and the Ornithodelphia. 



