TRUE TEETH AND HORNY PLATES OP ORNITHORHYNCHUS. 41 
C— 
Pig. 12. — The twenty-first section. The figure indicates that the section 
passes through the apex of the large anterior inner cusps on both sides. This, 
together with many previous figures, proves that the anterior slope of the 
large cusps is much steeper than their posterior slope, for many of the latter 
are seen in the sections, while at the first, or at most the second section 
through the anterior slope the apex of the cusp is reached. On the right side 
the papilla is not continuous. The small outer cusp is seen on both sides in 
its special compartment of the enamel organ, and the entrance of the papilla 
is shown in both. These inner cusps are much smaller than those of the 
second tooth (b), and their dentine is not nearly so thick, and the enamel is 
very thin indeed and is not represented in the figures. The twenty-second 
section is incomplete, so that neither of the teeth can be seen. 
Figs. 13 and 14. — The twenty-third and twenty-fourth sections. These 
show the appearance of the posterior slope of the large inner cusps cut 
through at two levels ; the small outer cusps are very distinct in both. 
Fig. 15. — The twenty-fifth section, the last figure. The posterior inner 
cusp is faintly seen through the enamel organ. The section passes between 
the two large inner cusps (as in Fig. 9 for the second tooth), but the anterior 
slope of the posterior cusps are seen from the surface. These are not covered 
with dentine (compare the posterior outer cusp of the corresponding tooth in 
the lower jaw, Fig. 10, c, from which dentine is also absent). In the five 
remaining sections (twenty-sixth to thirtieth) in which traces of the teeth 
appear, the posterior parts of the last teeth are seen ; but nothing is gained 
by figuring them. The twenty-ninth is the last section in which the eyes 
appear. It is clear that the third tooth is considerably smaller than the 
second (compare Fig. 16). 
Fig. 16. — X 9. Two teeth in the lower jaw, corresponding to the second 
and third (b and c) upper teeth, shown in the above-described sections. 
The piece of jaw from which the preparation was made ended abruptly an- 
teriorly (in the direction of the arrow), so that the presence of an anterior 
tooth (corresponding to a in the sections) could not be ascertained. How- 
ever, some sections of the opposite inferior maxilla render it probable, although 
not certain, that the tooth is present. The superficial structures (epithelium, 
mucosa, and enamel organ) were dissected away so as to expose the upper 
surface of the teeth. The inner side of the anterior tooth (b) had been 
previously cut so that all the inner cusps, except the anterior one, are removed, 
and the tooth papilla, enamel organ, and sub-epithelial tissues are seen in section. 
The enamel organ was easily removed from the surface of the teeth, except at 
the upper parts of the calcified cusps to which it strongly adhered, doubtless due 
to the formation of enamel in this region, and the consequent adherence of the 
inner layer of the enamel organ. The teeth are drawn from above, and from 
the inner side. The anterior tooth (b) is much the larger. Its large outer 
cusps (a. o. c. and p. o . c.) are calcified and hard over the region indicated by 
