HISTORY OF THE MARSUPIALIA 629 



the angle of the lower jaw, which is turned inward, or inflected, 

 at nearly a right angle with the body of the jaw. It is true 

 that one existing Australian genus has lost this character; 

 and in some of the placental orders, especially the Rodentia, 

 a somewhat similar structure may occasionally be found, 

 but it is never quite the same as in the marsupials, in which 

 it goes back to a remote antiquity. 



There are very constantly 19 trunk- vertebrae, of which 

 usually 13 are dorsals. The tail differs greatly in length in 

 the various genera, but most of them have well-developed tails. 

 An additional pair of elements, besides the three which are 

 found in the placentals, enter into the composition of the hip- 

 bones ; these are the marsupial bones, slender, flattened rods, 

 directed forward in the abdominal wall and diverging in 

 V-shape. Save in a few genera, clavicles are present and of 

 full size. The humerus may or may not have the epicondylar 

 foramen, but the femur never has the third trochanter. The 

 feet vary greatly in form and structure, in accordance with 

 the habits, but there is a very widespread adaptation to an 

 arboreal life, and even in terrestrial and burrowing forms more 

 or less distinct traces of this arboreal adaptation may be noted. 

 This fact has led to a generally accepted inference that all 

 existing marsupials had an arboreal ancestry. 



The soft parts and more especially the organs of reproduc- 

 tion are likewise very characteristic, and one or two of these 

 peculiarities may be mentioned. (1) In the female, the vagina 

 is double and on the abdomen is the pouch, or marsupium 

 (which gives its name to the order), a hair-lined bag, opening 

 either forward or backward, which serves to carry the young 

 and into which the teats open. A considerable number of 

 species have lost the marsupium, while other species of the same 

 genera retain it, and there can be little question that its absence 

 is a secondary condition. (2) Except in one modern Australian 

 genus, the marsupials have no true placenta, and the young 

 are born in a very immature state, incapable of even swallowing. 



