32 
EDWARD B. POULTON. 
kindly provided by Professor Moseley, the horny plates having 
been treated with chromic acid in Australia, in 1874. All the 
other specimens made use of had been preserved in spirit or 
dried. 
Form and Position of the Horny Plates. — The form 
and relative position of the plates of the upper jaw are seen in 
PI. IV, fig. 1, and further details of the upper and lower 
posterior plates are given in figs. 2 b, and 3 b. Transverse 
vertical sections of the upper anterior and posterior plates are 
represented in figs. 4 and 5 respectively. These figures being 
fully explained in the description of plates it is unnecessary 
to enter into further details here, especially as the most 
important points must be again alluded to in considering the 
probable relation between the true teeth and the plates. The 
lower anterior plates have not been figured, for they are very 
similar to those of the upper jaw, the longitudinal ridge being 
also placed towards the outer margin and the furrow lying 
between it and the inner margin. 
Structure of the Horny Plates. — All the plates possess 
the same histological structure. They are simply thickenings 
of the oral epithelium, penetrated in many places by long 
slender papillae, each of which sends up from its summit a 
column of soft, deeply-staining cells, into the stratum corneum. 
The thickening which forms the plates take place in the 
stratum corneum, the stratum Malpighii being of normal 
thickness. The plates are of course continuous on all sides 
with the oral epithelium. These facts are at once apparent in 
cut and stained sections, but when the dry teeth are ground 
down, as in the usual method of preparation, the papillae and 
columns of cells dry up and cease to be distinguishable, for 
each papilla and column forms a single vertical tube full of 
air, which may be more or less displaced by the mounting 
fluid, so that the appearances differ greatly in different sec- 
tions and in different parts of the same section. Such dried 
and ground-down sections are represented in fig. 5 (vertical) and 
in fig. 9 (horizontal). The deceptive resemblance of the hori- 
zontal sections to bone, is chiefly due to the presence of air in 
