TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHUS. 43 
calcification. D. The dentine : probably entirely typical, for the fact that the 
number of tubules appears to be smaller than usual doubtless follows from 
methods of preparation or preservation. The dentinal fibres are seen iu Fig-. 
2 to be very numerous. The faint oblique striation iu certain parts is 
probably due to parallel cracks in the brittle calcified tissue when cut by the 
razor. The black spots represent the uucalcified “ iuterglobular spaces,” 
D'. The inner part of the dentine which stains deeply, probably on account of 
its recent formation. The boundary between this and the completely formed 
dentine {!).) is highly irregular, the outer margin of recently formed tissue 
being reticulate. In mammalian teeth of a similar degree of development this 
line of demarcation is usually parallel with the inner and outer surfaces of the 
dentine, and the younger tissue stains less deeply than the older, o. The 
layer of odontoblasts : the appearance is entirely normal. I could not 
determine whether the dentinal fibres arise from the superficial or from the 
deeper cells. P. The tooth papilla of normal structure and appearance, con- 
taining capillaries at c. 
Fig. 2. — X 405. A small part of a vertical section through one of the 
lower teeth of uncertain position. Iu this case the tissue had been decalcified 
in dilute nitric acid, the thin layer of dentine {!).) is stained equally deeply 
throughout and exhibits no trace of dentinal tubes. The fact that such tubes 
exist in normal number is, however, conclusively proved by the numerous 
dentinal fibres (o.f.) which have been drawn out of them, doubtless because 
of the shrinkage of the internal softer tissues. The fact that some of these 
fibres {o'./'.) are much longer than the thickness of the dentine must be due 
to the stretching of the former to far beyond their normal length, probably 
because their distal ends remained fixed iu the dentine during the process of 
contraction. Each fibre appears to arise from the apex of a minute conical 
process, projecting from the surface of the layer of odontoblasts (o.). This 
appears to support the opinion that the fibres arc at any rate in many cases 
derived from the superficial cells. The tissue of the papilla is seen at P. 
Fig. 3. — X 188. Transverse sections of the enamel prisms, seen in a section 
from the same tissue as that from which Fig. 1 was taken. The shape is seen 
to be irregularly polyhedral ; but the variations iu this respect and in size 
are considerable. The irregularity in size is further proved by the fact that 
the prisms drawn in Fig. 1 appear to be much more slender than those which 
are here represented. 
Fig. 4. — X 188. A vertical section through a portion of the upper sur- 
face of the enamel organ, viz. the surface which is nearest to the oral epi- 
thelium. The figure was drawn from one of Dr. Parker’s sections of the 
posterior upper tooth on the right side. The whole tooth is shown ( X 14'5) 
in Fig. 14 on Plate II. The mucosa forming the tooth-sac is represented 
at m„ and the highly irregular outer membrane of the enamel organ at o. m.; 
the cell-outlines are not indicated. The middle membrane of the enamel 
