284 EGG-LA YING MAMMALS. 
simple type of structure, and it is possible that certain imperfectly known 
mammals from the earliest Secondary rocks may turn out to be such missing links. 
In regard to the distinctive features of the Egg-laying Mammals as a subclass, 
it may be mentioned that they differ from all other members of the class in having 
but a single excretory aperture to the body; whence their name of Monotremes is 
derived. Then, again, in their skeleton the shoulder-blade (scapula), instead of 
forming the sole support for the arm, is connected with the breast-bone (sternum) by 
another plate-like bone termed the metacoracoid, in advance of which is a third 
element known as the coracoid; the metacoracoid being always present in the 
lower Vertebrates. Another resemblance to Reptiles is found in the presence of a 
T-shaped bone overlying the breast-bone, and collar-bones (clavicles), and known 

SKELETON OF DUCKBILL. 
as the interclavicle; such interclavicle being similar to that of lizards and certain 
other reptiles, and unknown among higher mammals. Another feature of these 
animals is connected with the milk-glands, which instead of opening by nipples or 
teats, communicate with the exterior by a number of small pores situated in a cup- 
like depression in the skin of the abdomen. Although there are many other 
peculiarities in the structure of these animals, if we add to the above that their 
brains are of an exceedingly low and simple type, and that their young are pro- 
duced from eggs, we shall not have much difficulty in understanding why they are 
referred by naturalists to a distinct subclass. It may be added that their skeletons 
possess “marsupial” bones similar to those of the Pouched Mammals. 
THE DUCKBILL. 
Family ORNITHORHYNCHIDZ. 
The duckbill, or duck-billed platypus (Ornithorhynchus anatinus), derives 
its name from the duck-like bill forming the extremity of the head. In length the 
adult male duckbill measures from 18 to 20 inches from the tip of the beak to the 
extremity of the rather short tail. The muzzle is expanded and flattened, and has 
both the upper and lower jaws invested with a blackish naked beak not unlike that 
of a duck. This beak is bordered by a naked sensitive skin, forming a lappet-like 
fold at the base of the snout ; the nostrils being situated near its anterior extremity. 
The depressed and oval-shaped body is covered with short, close, and somewhat 
mole-like fur, comprising both longer hairs and a woolly under-fur; its usual 
colour being deep brown, becoming paler underneath. There are no external conchs 
to the ears; and the eyes are small, and in the living state inconspicuous. The tail 
