TRUE TEETH AND HORNY PLATES OP ORNITHORHYNCHUS. 13 
paration. The superficial epithelium shows very little trace of 
its subsequent differentiation into the plates ; it is somewhat 
thicker than elsewhere, but there are extremely few isolated 
papillary elevations instead of the very numerous papillae 
which are so characteristic in a vertical section of the horny 
plate. In the upper jaw, the superficial epithelium just 
external to the anterior teeth is abruptly raised to a some- 
what higher level than the rest, suggesting the appearance 
of the plates ; and for a few sections, anterior to the most 
anterior tooth, this differentiation is continued, the epithelial 
ridge becoming more pronounced but narrower. In the 
majority of sections containing teeth there is, however, no 
marked alteration of level in the epithelium and only a greater 
thickness. It is possible that the anterior ridge represents the 
front part of the plates, differentiating especially early ; but 
however this may be, it is quite certain, from the relations to 
the skull, that the latter correspond to the epithelium covering 
the posterior as well as the anterior teeth. In many of the 
twelve anterior sections which contained teeth, there was 
present beneath this external epithelial ridge, an epithelial 
tube invaginated from the side, which in transverse section 
much resembled a very rudimentary enamel germ. The ap- 
pearance is probably deceptive, for longitudinal and oblique 
sections showed the existence of a tube, aud the transverse 
sections indicated a distinct, although partially obliterated, 
lumen between the invaginated and the other walls. It is pro - 
bably a gland duct, but it is unfortunate that the sections are 
often incomplete and unsuited for examination in this locality. 
In the lower maxillae the epithelium only remained over the 
teeth, and no raised ridge could be seen, while the relative 
position of the teeth to the entrance and course of the inferior 
dental nerve, as compared with the cleaned structure and with 
the adult jaw, clearly showed that here also the teeth cer- 
tainly develop in a wide, distinct alveolar furrow, which is 
subsequently occupied by the posterior horny plate. 
Form of the Teeth. — It is only possible to indicate 
the form of the three anterior teeth, for the fourth is in 
