310 MONOTREMES. 



having neither enamel nor bone, but being composed of a horny 

 substance only embedded in the gum, to which they are connected 

 by an irregular surface, in the place of fangs. When cut through, 

 which is readily done by a knife, the internal structure is fibrous, 

 like nail ; the direction of the fibres is from the crown downwards.'* 

 Philos. Trans. 1802, p. 71. PI. ii. fig. 2. These constituent vertical 

 fibres are rightly described by Heusinger as hollow tubes, forming 

 a texture closely similar to that of Baleen and Rhinoceros-horn(l). 



In the figure which is appended to Home's description, two 

 other horny teeth are accurately represented " in situ," in the 

 lower jaw, and the corresponding teeth of the upper jaw have 

 been subsequently figured and described by F. Cuvier(2). The 

 horny teeth of the Ornithorhynchus are, in fact, eight in number : 

 the anterior tooth in the upper jaw is situated on the slender anterior 

 termination of the upper maxillary bone ; it is much extended from 

 behind forwards, and pointed at each extremity, but is low, very 

 narrow, and four-sided : the broadest side forms the base of 

 attachment, and is slightly concave and unequal : the outer and 

 inner exposed facets converge to a serrated edge in the young 

 Ornithorhynchus, but this becomes worn down to a flattened 

 surface, which forms the fourth side of the tooth in the old animals. 

 The corresponding tooth in the lower jaw (3) is rather narrower, 

 and retains longer its trenchant edge : these teeth hold the position 

 of the canines and premolars in the normal Mammalia. 



At a distance behind the anterior tooth equal to its own 

 length, is situated the horny molar, consisting of a flattened plate, 

 of an oblong subquadrate figure, bounded by a convex outline 

 externally, and a straight line internally, with the inner and an- 

 terior angle slightly produced: a slightly raised margin includes 

 two large concave surfaces a little elevated above the intervening 

 part of the grinding surface. The corresponding tooth of the lower 

 jaw(4) is narrower in proportion to its antero-posterior extent, with 

 a convex inner border, and has the two concave surfaces a little 

 more elevated. These surfaces, in both upper and lower molars, 



(1) Histologic, 4to. 1822, p. 197.— PI- 76, fig. 1, a and h. F. Cuv. 



(2) Loc. cit. PI. 83, p. 202. (3) PI. 76, fig. 2, u. (4) lb. fig. 2, b. 



