30 
MONOTKEMATA. 
In front of the beak, the free membrane ( d , fig. 2) measures 
half an inch in width, and at the sides rather less. At the 
base of the beak the skin forms a kind of fold or lappet, 
which averages about half an inch in width; at the angle of 
the mouth it is narrower, but immediately below the angle its 
width is nearly three quarters of an inch. This fold falls 
back over the loro part of the head and throat, and no doubt 
serves to prevent the mud, in which these animals are con¬ 
stantly grubbing in search of their insectivorous food, from 
soiling the fur: it may also serve to protect the eyes both 
from the mud and from the soil, when burrowing iu the 
ground. The nostrils are moderately large, and situated near 
the extremity of the upper surface of the beak. The lower 
mandibles are covered, like the upper, with a naked skin, but 
here it is generally of a whitish colour; the free, marginal 
portion is narrower, and on the upper surface it presents nume¬ 
rous transverse ridges, which may be compared to the ridges 
observed in the beak of a duck, and no doubt answers a similar 
purpose — that of sifting or straining the water from the 
materials collected in the mouth, which is moreover provided 
with two tolerably large cheek-pouches for their reception. 
“ An air-breathing, warm-blooded animal, which obtains its 
food by the capture of small aquatic animals (observes Prof. 
Owen) while submerged, must derive great advantage from 
the structure which enables it to transfer them quickly to a 
temporary" receptacle, whence they may be extracted and mas¬ 
ticated while the animal is floating on the surface, or at rest 
in its burrow.” 
The teeth in the Omithorliynchus, unlike those of most 
mammalia, are of a horny substance: they are eight in num¬ 
ber; lour are situated (one on each side of each jaw) in the 
lore part ol the jaws, and are in the form of long and narrow 
strips, being about three-quarters of an inch in length, und 
-n;-th of an inch in width: they present a slightly trenchant 
