TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHUS. 39 
P. Tooth papilla. o. Layer of odontoblasts, i. c. Inner chief cusp. o. c. 
Smaller outer cusp. N. Epithelial nodule in the middle layer of the enamel 
organ, almost immediately over the chief cusps of the two posterior teeth. 
The references have not been unnecessarily repeated on all the figures. 
A— 
Fig. 1. — This was the first section in which teeth appeared, and here only 
on the right side. The shape is long and narrow, and the apex is nearly 
in contact with the lower surface of the oral epithelium. The direction 
of the tooth is obliquely downwards and inwards. The dentine is thick, and 
a thin layer of enamel is present : the former is cut tangentially towards the 
base of the tooth, aud therefore it appears irregularly interrupted by the 
tissue of the papilla and odontoblasts. 
Fig. 2 represents the condition of the next, the second section. The 
anterior tooth is now seen on the left side, and much resembles the section 
just described, except that the papilla is only fairly reached at the apex of the 
tooth. On the right side the tooth is broader, aud indicates a tendency 
towards the formation of two cusps. The entrance of the papilla is seen, 
although the outer layer of the enamel organ crosses this part of the section 
at a different level, and can be seen by focussing. Much of the dentine is cut 
tangentially. The very complete investment of the papilla by dentine is 
noteworthy, especially when compared with the two posterior teeth. In this 
and the next figure the dentine is seen to be formed round the very base of 
the tooth, so that the entering papilla is encircled by it. 
Fig. 3 represents the third section ; the existence of two cusps seems 
especially clear on the right tooth. The fourth section is not figured : on the 
right side the epithelial process ( d . p.) aud the tangentially cut enamel organ 
are all that can be seen ; on the left side the base of the tooth is cut tangen- 
tially. The fifth and sixth sections only show the diminishing epithelial process 
on both sides. The seventh aud eighth sections show the tangentially cut 
enamel organ of the second tooth. 
B— 
Fig. 4 represents the appearance in the ninth section from the most anterior 
in which a tooth was seen. The dentine is cut tangentially towards the base 
on both sides, the apex represents one of the chief inner cusps ; ou the left 
side this is cut through at its highest point, aud on the right side very nearly 
so. An epithelial nodule (N.) is seeu in section above the apex ; it may be 
connected with the inner layer of the enamel organ above the summit of the 
tooth. A thin layer of enamel is present. 
F'ig. 5. — The tenth section : the shape of the large inner cusps is well seen 
on both sides. The layer of odoutoblasts ( o .) is distinct. The small outer 
cusps are indicated by the direction of the inner layer of the eDamel organ (1) 
on the outer side of the papilla (P.). In this vertical section the anterior 
part of both eyes arc also cut through for the first time, and it is thus seeu 
