SS ON CLASSIFICATION. 



ledge, but I shall direct your attention at once to that important 

 step towards dividing the Mammalia into large groups, which 

 was taken by the eminent French anatomist, M. de Blainville, 

 so far back as the year 1816. M. de Blainville pointed out that 

 the Mammalia might be divided into three primary groups, 

 according to the character of their reproductive organs, espe- 

 cially the reproductive organs of the female. He divided them 

 into " Ornithodelphes," " Didelphes," " Monodelphes ;" or, as we 

 might term them, Ornithodelphia, Didelphia, Monodel- 

 phia. Xow, 1 do not mean to assert that M. de Blainville 

 defined these different groups in a manner altogether satis- 

 factory, or strictly in accordance with all the subsequently dis- 

 covered facts of science, but his great knowledge and acute 

 intuition led him to perceive that the groups thus named were 

 truly natural divisions of the Mammalia. And the enlargement 

 of our knowledge by subsequent investigation seems to me, in 

 the main, only to have confirmed De Blainville's views. 



The division of the Ornithodelphia comprises those two 

 remarkable genera of Mammals, as isolated in geographical 

 distribution as in structure, — Ornithorhynclius and Echidna, — 

 which constitute the order Monotremata. 



In these animals the angle of the lower jaw is not inflected, 

 and the jaws are devoid of true teeth, one of the two genera only 

 (Ornithorhynclius) possessing horny plates in the place of teeth. 

 The coracoid bone extends from the scapula to the sternum, 

 with which it is articulated, as in birds and most reptiles, and, 

 as in many of the latter, there is an episternal bone. There is 

 no marsupial pouch, though bones wrongly termed "marsupial" 

 are connected with the pelvis. But it is to the structure of the 

 female reproductive organs that the Ornithodelphia owe their 

 name. The oviducts, enlarged below into uterine pouches, but 

 opening separately from one another, as in oviparous vertebrates, 

 debouch, not into a distinct vagina, but into a cloacal chamber, 

 common to the urinary and genital products and to the faeces. 

 The testes of the male are abdominal in position throughout 

 life, and the vasa deferentia open into the cloaca, and not into a 

 distinct urethral passage. The penis is indeed traversed by an 



