20 
EDWARD E. POTTLTON. 
represented diagramraatically. All the four layers are very 
distinet ; the enamel cells ( e . c.) are of the normal columnar 
type, the stratum intermedium (s. i.) is very thick, and the 
stellate reticulum (in. in.) is typical although of no great 
thickness ; the cells of the outer epithelium ( o . m.) have 
already lost their primitive columnar appearance, and are 
somewhat flattened. There is a distinct and typical “ neck ” 
(not shown in the figure), continuous with the oral epithelium. 
The tooth-sac and papilla are also normal, and in fact the 
whole structure is in every way characteristic of an early stage 
in the development of a mammalian tooth. 
Conclusions. — It has been already stated that the teeth 
of Ornithorhynchus are typically mammalian. The two chief 
and largest teeth seem to me to resemble closely the mnlti- 
tuberculate molar teeth of Myrmecobius. In the lower jaw 
the resemblance is very striking, nearly all the lower molars 
of this animal having four small internal cusps and two ex- 
ternal cusps, the only difference being in the fact that the 
internal cusps are the higher, while the outer are higher in 
Ornithorhynchus. In the upper jaw nearly all the molars of 
Myrmecobius also have fewer (2 — 3) cusps on the internal 
edge, and more numerous (4 — 6) cusps on the outer edge, and 
the relative height is also the reverse of that found in Ornitho- 
rhynchus. 
In addition to the confirmation of the predictions quoted at 
the beginning of this paper, the typically mammalian character 
of these teeth confirms in the most striking manner an opinion 
expressed by Dr. Parker and Mr. Oldfield Thomas as to the 
ancestry of the Edentata. Thus the latter writes (1. c. p., 458) : 
*‘In the Edentata on the other hand, we find, as is well known, 
characteristics wholly at variance with those of all other 
mammals. In fact a study of the teeth of this order soon 
induces a belief that the variance is so great as to preclude the 
possibility of the Edentates lying within the same lines of 
development as other mammals, a belief that tallies exactly 
with the conclusions of Professor Parker (‘Phil. Trans.,’ 1885, 
p. 116, ‘Mammalian Descent/ p. 97, 1885), drawn from the 
