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 still 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 

 slab of cartilage on which the dentary bone grafts itself to 

 form the bulk of the solid niaxilla 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 Orinthovhynchus. 

 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 well- 



