I 4 NATURE STUDIES. 



The jaws of the Ornithorhynchus are prolonged to 

 form a flattened horny bill (Fig. 6), on the upper 

 aspect of which the nostrils are seen. It is the pos- 

 session of this bill which, has given origin to the name 

 " duck-billed," applied to this animal. The Echidna 

 possesses no such structure, but has simply a flexible 

 .snout. 



There also exist in the internal anatomy of these 

 curious animals certain characters which relate them 

 to the birds and reptiles. For example, the bones of 

 the head are firmly ossified together, as in birds, and 

 the sutures, or lines of union of the skull-bones, do 

 not persist, as is usually the case with quadrupeds, 

 whilst the hollow of the haunch-bones (Fig. 7, d), in 

 which the head of the thigh works to form the hip- 

 joint, is not fully ossified, and ttyus comes to resemble 

 the similar structure in birds and crocodiles. The ears 

 of these lower quadrupeds differ from those of other 

 mammals in not possessing a spiral arrangement of 

 that part of the organ named the cochlea. As in very 

 many reptiles, the upper, or front, or neck-ribs of 

 these quadrupeds long remain as separate bones; and 

 the same remark holds good of the curious little pivot 

 (odontoid process) on which the head turns. This 

 pivot in quadrupeds is firmly joined to the second 

 bone of the neck; but in the Monotremes, as in 

 reptiles, it remains separate and distinct till a very 

 late period if, indeed, it becomes ossified at all. 



The internal anatomy likewise reveals characters of 

 bird and reptile life which can only be alluded to here. 



