TRUE TEETH AND HORNY PLATES OE ORNITHORHYNCHUS. 35 
distinctly traced on the twelve anterior sections containing 
teeth, and it gradually disappeared in a few sections posterior 
to the twelfth. Anteriorly to the teeth it became narrower, 
but more defined, becoming most distinct in the sixth, seventh, 
and eighth sections in front of the first section, which ex- 
hibited traces of a tooth (see PL II, fig. 1). In front of the 
eighth section it rapidly disappeared. The epithelium of the 
right ridge in the above-mentioned seventh section is shown 
in vertical transverse section in PI. IY, fig. 13, and the ap- 
pearance strongly suggests an early form of the plate. Traces 
of papillae are visible beneath the ridge, and the greater thick- 
ness of the epithelium is very noticeable. 
I believe that the following account will be found to ex- 
plain the relation between the teeth and plates, and to re- 
concile the apparently conflicting observations. The calcified 
true teeth of Ornithorhynchus became unsuited to the needs 
of the animal when it adopted a mode of life in which large 
quantities of sand were necessarily taken into the mouth with 
the food, when in fact it first fed upon insect larvse, &c., which 
it dug with its bill out of the mud and sand at the bottom of 
streams. The fact that large quantities of sand are introduced 
with the food has been already proved, and I have noticed that 
the concavities of the posterior plates are sometimes filled with 
mud, sand, and the debris of food. At the same time the 
presence of sand may be valuable in assisting to grind down 
the food, and it is possible that a store is kept in the cheek- 
pouches for this purpose, and is intentionally added during 
mastication. Under such circumstances two things might 
happen : the true teeth might be protected from the effects of 
wear by continuous growth from persistent pulps or by a con- 
tinued succession ; or a constantly growing horny plate might 
be developed from the oral epithelium, and might be substi- 
tuted for the true teeth. While a corresponding difficulty has 
been met by the first method almost universally among Mam- 
malia, we must remember that there is no a priori reason why 
this should be the case. Natural selection only demands rela- 
tive success and feasibility, and the means by which such 
