1456 EGG-LA YING MAMMALS, OR MONOTREMES 



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 breastbone (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 breastbone, and collar bones (clavicles), and known 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 



SKELETON OF DUCKBILL. 



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 produced 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 ORNITHORHTNCHID^ 



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 eighteen to twenty 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 hair and a woolly under- 

 fur; its usual color being deep brown, becoming paler underneath. There are no 



