122 MONO TREMA TA 



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 1 announced the discovery 

 in an embryo of teeth which were regarded as quite functionless. In 

 the following year, however, Mr. 0. Thomas 2 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 among 

 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. 353 (1888). 

 - Ibid. vol. xlvi. p. 126 (1SS9). 



