TRUE TEETH AND HORNY PLATES OP ORN1THORHYNCHUS. 19 
One peculiarity of this layer is the presence of an epithelial 
nodule situated just beneath the outer layer of the enamel 
organ, almost immediately over the apex of each calcified 
cusp of the second and third tooth (see PI. II, n, in figs. 4, 
5, 11, and 12). Nothing of the kind could be made out in the 
case of the first upper tooth. In thin sections of the lower 
teeth, prepared for histological examination, the nodule was 
repeated in many sections, although only shown once or twice 
in the consecutive sections of the upper teeth represented on 
PI. II (for these were prepared for morphological rather than 
histological woi’k). In some cases there was the appearance 
of an epithelial cylinder extending from the nodule towards 
and perhaps reaching the stratum intermedium or enamel cells 
over the apex of the cusp. It seems clear that the nodule is 
in some way associated with the chief cusp, for there was always 
a nodule above each of the latter, while they were never found 
elsewhere. Further material and probably other stages will 
be necessary in order to make out the significance of these 
structures. The minute details are shown in PI. II, fig. 6, n, 
where it is seen that the inner cells appear to be corneous and 
collected into a dense central mass, between which and the 
outer fusiform cells is a space containing loosely-packed cells 
resembling the former in character. The position at the 
extreme edge of the stellate reticulum is also shown. 
7. The Outer Membrane of the Enamel Organ or 
External Epithelium. — This appears to be of normal cha- 
racter ; it is a highly irregular and apparently discontinuous 
layer. Its structure and appearance is sufficiently indicated 
in PI. Ill, figs. 4 and G, o. m. I could not detect the presence 
of a persistent “neck” continuous from this layer into the 
oral epithelium but the condition of the less developed tooth, 
to be described below, indicated that such a neck had 
existed. It is possible that the process of the superficial epi- 
thelium shown in PI. II, figs. 1, 2, 3, d. p., is a remnant of 
the “ neck.” Sec description of Plate. 
The Less Developed Fourth Tooth. — The appearance 
is shown in PI. Ill, fig. 7 x 50, the enamel germ being 
