TRUE TEETH AND HORNY PLATES OP ORNITHORHYNCHUS. 15 
The third tooth, both above and below, differs in the fact 
that the anterior large cusp is alone calcified, although the 
posterior cusp is present. Furthermore, the anterior cusp is 
not so large or so thickly calcified as those of the second teeth. 
Figure 16, c, shows that five inner cusps are present in the third 
lower tooth, the central one being very minute. The existence 
of the same cusps on the outer side of the upper teeth is shown 
in c, figs. 12 — 15, o. c. Further details of the form of the 
upper teeth can be learnt from figs. 1 — 15 on PI. II, and in 
the description of the plate. The shape of these teeth is 
characteristically mammalian, and, together with their posi- 
tion, points to correspondence with some part of the molar 
series of other Mammalia. 
Structure of the Teeth. — The structure is also charac- 
teristic of mammalia. The tissues of the three anterior teeth 
will be considered from within outwards. 
1. Tooth-papilla or Dentine Germ. — This is indicated 
at p in many of the figures in Plates II and III ; its structure 
being entirely normal, and the same may be said of the layer of 
odontoblasts (o.) which form its superficial part wherever 
dentine is developed. These are well seen in PI. Ill, figs. 1 
and 2, and in the latter figure the papilla has shrunk, and has 
therefore drawn the dentinal fibres (o./.) out of their tubes in 
the dentine. Some of the fibres ( o' .f '.) remaining fixed in the 
latter tissue have become stretched to far more than their 
normal length. I could not determine whether the fibres are 
processes of the superficial odontoblasts or of deeper cells (as 
stated by Klein), but the appearance of a conical process 
with its apex continuous with the fibre, seemed to support the 
former more common view, for, at any rate, the majority of the 
fibres. 
2. Dentine. — In most cases the tissue appeared homogen- 
eous, but this was a result of decalcification, for sections of 
tissue which had not been so treated gave the usual appearance 
(PI. Ill, fig. 1, d). I have sometimes noticed the same 
homogeneity in the decalcified teeth of higher mammals. 
Although the dentinal tubes did not appear to be very mime- 
