46 
EDWARD B. POULTON. 
is excavated to a muck greater depth than the others. The divisions between 
the compartments are lower than the borders of the plate. 
Fig. 3. — Natural size. A. The socket of the right lower posterior plate> 
seen from above. The concavity is in this case somewhat distinctly compart- 
mented to correspond with the two chief parts of the plate. Each compart- 
ment is roughly divided into four small concavities. There is also a very 
distinct posterior and internal small compartment for the corresponding part 
of the plate. The bone is pierced by numerous foramina, leading into the 
very large canal for the inferior dental nerve. B. The surface of the right 
lower posterior horny plate, as seen from above. The outer border is here 
thicker and somewhat higher than the inner, but the highest part is the 
anterior border, and especially its inner part ; the posterior border is also higher 
than the lateral borders. The third small concavity is here posterior in position, 
and it is not excavated so deeply as the others, and its edge is somewhat 
higher than the adjacent borders. The divisions between the compartments 
are lower than the borders of the plate. A comparison of the relative heights 
of the borders and other parts of these plates will show that they are very 
well adapted for interlocking in mastication, an antero-posterior motion being 
especially favoured. 
Fig. 4. — X 24 - 5. A transverse vertical section through the left upper 
anterior horny plate. The section was taken towards the posterior end at 
the maximum breadth of the plate. The ridge and furrow are seen in section. 
The structure is obviously entirely epithelial, and passes into the oral epitke- 
thelium on both sides. The section was cut and stained, and the stratum 
Malpighii is clearly shown in the lower part, and the stratum corneum in the 
upper. Numerous fine papilla; enter the former, and each of them sends up 
from its summit a long column of soft, deeply-staining cells into the stratum 
corneum. These columns reach the surface, and doubtless largely determine 
the relative wear of the plate. Thus they are absent or very minute in the 
ridge, and especially large in the furrow. 
Fig. 5. — x 9. A similar section through one of the upper posterior plates ; 
the slight elevation in the concavity being doubtless an oblique section of the 
low ridge which separates the two concavities. This section had been ground 
down and dried, so that the papilla; and columns of cells have dried up, their 
place being occupied by air which causes the dark appearance. The raised 
borders have only very minute papilla;, &c., in some places. The continuity 
with the oral epithelium is also seen. 
Fig. 6. — X 24 - 5. A horizontal section through the stratum corneum of 
the plate shown in Fig. 4. The section was cut and stained, and shows the 
columns of soft cells in transverse section. The position of the ridge and the 
furrow can be determined by the size of the columns. 
Fig. 7. — X 50. A horizontal section through the stratum corneum of the 
concavity of the plate shown in Fig. 5. The section was cut and stained, 
