Xect. II.] THE EGG-BEEAKING BEAK. 45 



margins of the beak; for in that bird, as in its con- 

 .geners, the bones of the upper face run close to the 

 quick that secretes the bony sheath. But the duck- 

 billed mammal is quite unique ; the whole outline of ' 

 the great rostrum is formed by a large sheet of solid 

 hyaline cartilage right and left. Over this, in front, 

 the thin horny layer stUl shows the " neb " for breaking 

 the egg-shell, quite like what is seen in Tortoises, Croco- 

 diles, and Birds. 



The extraordinary growth of true cartilage in the 

 •extended upper lip is quite similar to the growth 

 in the lower lip of mammals generally, namely, that 

 ^lab of cartilage on which the dentary bone grafts itself to 

 form the bulk of the solid maxilla inferior, or lower 

 Jaw. 



We have to go down, as I have just stated, amongst 

 the lower cartilaginous fishes for similar growths of 

 superficial cartilage in the region of the mouth. But 

 although I am quite familiar with superficial cartilaginous 

 structures in these fishes, it is only in the Tadpoles of the 

 Frog and Toad, and in the adult Lamprey, that I find 

 anything equal to what is seen in the Ornithorhynchus. 

 In those pouch-gilled (marsipobranch) types, however, 

 these parts are all separate, neat, finished tracts of 

 -cartilage, each having its place and its function as an 

 -orderly element of the front face. But in this strange 

 remnant of a lost race of archaic mammals, the growth 

 of cartilage is a wild leafy tract, very unlike the weU- 



