22 
EDWARD B. POULTON. 
ancestral condition of the mammalian organs of mastication. 
I shall shortly have occasion to show that the very ancient 
hairy covering of Mammalia is also greatly modified in the 
Monotremes. As above stated the facts here set forth strongly 
confirm the identification of the mammalian tooth with the 
placoid scale. 
Again, I have been enabled to suggest a possible explanation 
as to the meaning of the largely developed middle layer of the 
enamel organ — the stellate reticulum — which is so character- 
istic of Mammalia. The condition of these structures in 
Ornithorhynchus clearly indicates that the association of such 
a peculiar tissue with teeth of a mammalian form must be very 
aucieut. Tomes (1. c. p. 125, 126) in describing the tissue 
states, “ It has been supposed to have no more important 
function than to fill up the space subsequently taken up by 
the growing tooth. ” I think that a little consideration will 
show that such a function may be extremely important. It is 
clear from the method of tooth formation, in which the oldest 
dentine is the superficial crust, and all additions are upon the 
inside, that the shape of a mammalian tooth, so far as it is 
represented by the contour of the dentine, must be modelled 
beforehand iu the soft tissue of the papilla or dentine-germ. 
This is well seen in PI. II, fig. 16, c, in which the shape of 
the tooth is obvious, although only a very small part of the 
surface is calcified. When the subsequently formed tooth is 
to be merely conical or of some other simple shape such as is 
found in Vertebrata other than Mammalia, there is no reason to 
suppose that the dentine-germ would encounter any difficulty 
in assuming such a shape, although subject to the resistance of 
the dense subepithelial tissues. But the case is different when 
the soft papilla is compelled, as in Mammalia, to assume a 
complex tuberculate outline; and hence I believe arose the 
necessity for the existence of a superincumbent tissue of gelati- 
nous consistence, which would exert a pressure only a little 
greater than that which is necessai’y to keep the enamel cells 
iu contact with the growing papilla. As a test of the value of 
this suggestion, it will be of interest to compare the size 
