36 
EDWARD B. POULTON. 
success is attained is entirely determined by the character of 
the variations which appeared at the critical time ; so that 
there is no difficulty whatever in believing that the case has 
been met by continuous growth in many instances, and by the 
substitution of another continuously growing tissue in other 
instances. Ornithorhynchus is not the only example of this 
method among Mammalia. A similar difficulty, doubtless 
also caused by the presence of sand and mud in the food, has 
been met in the same manner in the case of the lower incisors 
of the Sirenia, which are completely functionless and covered 
by a horny plate. In the Manatee these true teeth are absorbed 
early, as in Ornithorhynchus, but in the Dugong they persist 
until old age, thus proving the entirely independent origin of 
the horny plate. 
We must assume that the chief dental area has been made 
use of continuously throughout the whole period of change, 
for the plates are found to occupy the exact position of the 
teeth. In this manner the muscular and other arrangements 
upon which the movements of mastication depend would also 
remain unchanged. We may suppose that the rapidly-wearing 
true teeth were at first reinforced by an adjacent corneous epithe- 
lial thickening in the position of that described in Dr. Parker’s 
sections (Plate IY, fig. 13), and that the thickening gradually 
extended over the young true teeth, so that these, instead of 
piercing the epithelium, merely conferred the shape of their 
crowns upon the latter. Each true tooth was in fact protected 
by an additional indurated layer external to the enamel. At 
first the teeth may have been thus protected during the earlier 
part of the animal’s life, coming eventually to the surface. 
This would take place at successively later periods until they 
ceased to appear altogether. In strong support of this inter- 
pretation is the fact, already quoted, that the two chief con- 
cavities of the plates arise separately and fuse at a later 
period. Each of these separate tubercles would, according to 
this theory, correspond to one of the two chief teeth in each 
jaw. A section across the specimen described by Sir Richard 
Owen would probably settle the question, The anterior small 
