122 MONOTREMATA 
and molariform. The upper surface of the lateral edges of the, 
mandible has also a number of parallel fine transverse ridges, like 
those on the bill of a Duck. Until 1888 it was thought that true 
teeth were totally wanting throughout the life of this animal ; but in 
the spring of that year Mr. E. B. Poulton! announced the discovery 
in an embryo of teeth which were regarded as quite functionless. In 
the following year, however, Mr. O. Thomas ? was fortunate enough 
to find some young skulls with functional teeth in situ, and was thus 
enabled to give a detailed account of their structure and of their 
relations to the cornules. From these specimens it appears that 
the teeth are functional for a considerable part of the life of the 
animal, cutting the gum in the usual manner, and, after being worn 
down by friction with food and sand, are shed from the mouth 
in the same manner as are the milk-teeth of other mammals. The 
cornules are developed from the epithelium of the mouth under and 
around the teeth, and the hollows found in the middle of them are 
the vestiges of the alveoli from which the teeth have been shed. 
One of the skulls showed on either side, both above and below, two 
completely calcified teeth ; but in another example there were three 
teeth on either side of the lower jaw. According to Mr. Thomas’s 
account, ‘the teeth themselves are broad, flat, and low-crowned. 
The upper ones have each two high, conical, internal cusps, from 
which minute ridges run downwards and outwards to the outer 
borders of the crowns, where the edge is peculiarly crenulate rather 
than cuspidate, in the ordinary sense of the word. On the whole, 
the anterior and posterior upper teeth are essentially similar to one 
another, except that the former are narrower, and their outer edges 
are less markedly crenulated. In the lower jaw there is a greater 
difference between the two. The anterior is triangular in outline, 
its longest side is placed antero-externally, and its anterior and 
postero-external angles have each a high pointed cusp, ridged on 
its internal aspect, while the posterior and internal borders are 
indistinctly crenulated. The posterior tooth is broadly quadrangular 
in outline, with a projecting antero-internal angle. As in the cor- 
responding tooth above, there are two cusps on one side, and a series 
of crenulations on the other, but they are of course reversed, the 
cusps being external and the crenulations internal, The cusps are 
high, and connected with transverse ridges running across towards 
the internal border.” 
In trying to find any teeth like those of the Duck-bill amone 
other known mammals Mr. Thomas considers, as was first suggested 
by Professor Cope, that those of the Mesozoic Multituberculata (p.109) 
make the nearest approximation. He adds, however, that “it must 
be insisted that the resemblance between the Multituberculate 
1 Proceedings of the Royal Society of London, vol. xliii, p. 358 (1888), 
2 bid. vol. xlvi. p. 126 (1889). 
