42 
EDWAED B. POULTON. 
the red colouring, the calcification terminating below in a sharp line of demar- 
cation rendered especially distinct from the fact that the jaw had been faintly 
stained as a whole in carmine. The tooth is seen in vertical section at P., 
the reference mark being placed upon the entrance of the tooth papilla. The 
enamel organ and sub-epithelial tissues are also seen in vertical section at e'. o'. 
and s'. ml. respectively, and the same tissues are also seen in horizontal 
section at e. o. and s. m. The line l indicates the boundary between enamel 
organ and sub-epithelial tissue. Between the two teeth (b and c) at the 
point x. the two enamel organs appear to become fused. I could not, however, 
feel sure upon this point. The entire crown of the smaller posterior tooth is 
shown, and it is seen that there are four small inner cusps (the reference being 
to the anterior one) besides a very minute fifth cusp. The large anterior outer 
cusp {a. o. c.) is calcified, but the smaller posterior outer cusp ( p . o. c.) 
remains soft like the inner cusps. Part of the inferior maxilla is shown at B. 
The appearance of these teeth strongly confirms the conclusions as to relative 
size, shape, and structure, drawn from the sections of corresponding teeth in 
the upper jaw (Figs. 4 — 15). The relative position of the large and small 
cusps on the upper and lower teeth respectively is an obvious remnant of the 
time when the surfaces of the upper and lower teeth fitted together for the 
performance of mastication. 
PLATE III. 
Fig. 1. — X 188. A portion of a vertical section through one of the 
developing teeth of Ornithorhynchus. The tissue had not been decalcified, 
and hence the structure of dentine and enamel is better shown than in other 
cases. The teeth of which the structure is shown were contained in an 
isolated piece of tissue, probably removed from the lower jaw. The relative 
position of the tooth from which the section was taken is uncertain, but the 
histological details are evidently quite typical, m. m. The middle membrane 
of the enamel organ, made up of a honey-combed reticulum of cell-plates. 
Capillaries are present in it, but could not be seen in the part of the section 
figured, s. i. Stratum intermedium of Hannover : the outlines of the small 
spherical cells are not indicated, e. c. The enamel cells : long and columnar, 
bounded by a sharp and almost straight line of demarcation from the last- 
mentioned layer, and separated by a very irregular line from the enamel prisms. 
E. The enamel prisms : the layer is faintly marked by fine closely placed lines, 
running parallel to the surface. The axes of enamel prisms are seen' to form 
an obtuse angle with the axes of the enamel cells, perhaps due to shrinkage. 
I could not determine whether each prism exactly corresponds to an enamel 
cell, although this is probably the case. The prisms are faintly stained round 
the projections of the last-mentioned layer, probably due to incomplete 
