234 NATURAL HISTORY. 



in life, or altogether. And the cavity for jointing of the thigh bone with the pelvis (the acetabulum) 

 is not perfect, there being a part of it not filled with bony matter. 



The ear is singularly simple in its construction in this sub-order, and the cochlea is not coiled into 

 a spiral ; moreover, there is no external ear. 



The hemispheres of the brain, which are convoluted in the Echidnas, are smooth in the Orni- 

 thorhynchus, and in both the central commissure, or corpus callosum, just exists, whilst the anterior 

 one is large. The Ornithorhynchus inhabits Van Diemen's Land and Australia, as far north as 

 Queensland inclusive. 



The Echidna and the Ornithorhynchus belong to a sub-order of the Marsupialia which, whilst it 

 has some structures resembling those of the sub-order of Marsupiata, possesses others which link it 

 with the birds and reptiles. This sub-order is the lowest amongst the Mammalia, and the animals 

 which are included in it have the following peculiarities : The marsupial bones are present, the 

 uterus is double, and the young are not nourished when within the parent by a placenta ; there 

 is no inflection of the lower jaw ; the shoulder-girdle has additional bones ; there are no true 

 nipples ; the teeth are either absent or horny ; the extei-nal ear is not present, and there is not a true 

 pouch. The excretion of the kidneys and the contents of the bowels fall into one receptacle, through 

 which the young also pass. It is called a cloaca, and receives the outlets of the rectum and 

 urogenital canal. The presence of the spur on the heel is also a peculiarity. 



A fossil Echidna has been discovered in deposits on the Darling Downs. 



The Echidnas form one genus and the Ornithorhynchus another, and the classification of the 

 whole is as follows : 



ORDER MARSUPIALIA. 



SUB-ORDER . . . MONOTREMATA. 

 Genus .... Echidna. 

 Genus .... Ornithorhynchus. 



In concluding this Natural History of the Marsupiata and Monotremata, it is necessary to direct 

 attention to the fact that they are less well defined than the o.^er great groups of the Mammalia. As 

 sub-orders, they are very artificial, for some of their most important structural peculiarities are deficient 

 in some of the genera. The inflected lower jaw is not invariably found, the marsupium does not 

 always exist, and the marsupial bones differ in shape and size, and are occasionally absent. With 

 regard to these bones or cartilages, they are not connected with the pouch, but with the muscular 

 system of the belly. They have been shown by Huxley to be present in Crocodiles, and in the 

 amphibian called the Salamander. Hence these structures are relics of a remote ancestry, and have 

 but slight functional importance. The authorities whence I have derived most of this Natural History 

 are Waterhouse, Owen, Huxley, Bennett, Gould, and Kreeft, to whom I am under great obligations. 



P. MARTIN DUNCAN. 



Postscript. Mr. W. H. Caldwell has discovered that the Monotremes lay eggs as has been sus- 

 pected, and he has traced the development of the animal during the hatching. The eggs resemble 

 those of a Reptile more than a Mammal, and the yolk is in such excess that it is not all sub- 

 divided during the early stages of the embryo of the Monotreme, but some remains over and above, 

 upon which it is nourished in the early stages within the egg. The heart of Ornithorhynchus 

 paradoxus is interesting because Meckel, and, subsequently, Prof. E. Ray Lankester, F.R.S., have 

 shown that it differs from that of the higher Mammalia, and is not without considerable resemblance 

 in some of the valvular structures to the hearts of Birds and Crocodiles. The last-named naturalist 

 states that the reflux of blood into the right auricle on contraction of the ventricle, is not pre- 

 vented by the presence of the special structure seen in Mammals, but by an arrangement which is 

 observed in Reptiles and Birds. The mitral valve is with three divisions instead of two, as in the 

 higher Mammalia, but the tendinous structures seen in this last group, to terminate the muscular 

 structures, do not exist in the Monotreme. This increase of the muscular structure is not so palpable 

 in Echidna. It is interesting to know that the special structures of the right side of the heart in the 

 adult Ornithorhynchus are seen in the embryonic condition of the higher Mammalia. 



