TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHUS. 45 
Eig. 7. — X 50. A section from the same lower jaw as that from which 
Fig. 2 was drawn. Behind and interior to the comparatively highly-developed 
posterior tooth there was a much earlier rudiment of a third or fourth tooth, 
also present in the upper jaw. The whole appearance exactly resembles the 
corresponding stage in the development of teeth in the higher mammalia. 
The oral epithelium is shown at e. p. Beneath this the young enamel organ is 
also coloured red, and its constituent layers are diagrammatically represented. 
The enamel cells ( e . c.) of its inner layer are of the normal columnar type ; 
externally to these the very thick stratum intermedium ( s . i.) is shown, and 
then again the beginning of a middle membrane at m. m., covered by an 
outer membrane (o. m.) of cells which have already lost their primitively 
typical columnar shape and have become somewhat flattened. The tooth-sac 
(«.) is clearly marked off from the surrounding mucosa and submucosa (*».), 
and continuous with it is the well-marked tooth papilla (P.), which ascends 
into the space formed by the invagination of the enamel organ. The “ neck ” 
of the enamel organ is not seen in the figure ; it is continuous with the oral 
epithelium, 
PLATE IV. 
Fig. 1. — Natural size. The right side of upper bill and palate seen from 
below, showing the relative position and form of the horny plates. The longi- 
tudinal ridge of the anterior plate is nearer to its outer margin, and between 
it and the inner margin there is a shallow furrow. The three concave surfaces 
of the posterior plate are plainly seen. The animal had not attained the full 
size, so that both plates arc smaller than usual, or perhaps the smaller size 
may be due to sex. Anteriorly, in the middle line, is an oblique furrow con- 
taining a canal which leads to the nasal passages, and on the inner side of its 
opening into the furrow a small but distinct tubercle is seen. Posterior to 
this are many curved corneous ridges. 
Fig. 2. — Natural size. A. The socket of the right upper posterior plate, 
seen from below. The concavity of the alveolus is seen to very roughly cor- 
respond to the two posterior concavities of the plate, while it possesses a small 
but distinct and well-separated anterior and internal compartment for the 
corresponding concavity of the plate. The bony wall is very thin over most 
of the alveolar surface, and it is seen to be perforated by numerous foramina 
through which vessels, &c., pass to the base of the horny plate. B. The 
surface of the right upper posterior horny plate as seen from below. The 
internal border is very thick, and reaches a much higher level than any other 
part ; and the anterior and posterior borders are much lower than the others. 
The small anterior and internal concavity is at a somewhat lower level, and 
