284 EGG-LAYING 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 Qrsithorhyxchidje. 



The duckbill, or duck-billed platypus {Ornithorhynchus anatimis), 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 



