TRUE TEETH AND HORNY PLATES OF ORNITHORHYNOHUS. 33 
the dried-up, softer, irregularly-shaped centres of the corneous 
cells which surround the columns. Such minute ramified air 
spaces bear considerable likeness to lacunae with branching 
canaliculi, and their arrangement relatively to the larger air 
spaces in the dried-up columns of soft cells is also somewhat 
suggestive of bone (see fig. 9, in which, however, the air of the 
large central spaces has been almost entirely displaced by the 
balsam). A vertical section, similarly prepared, at once dis- 
poses of the resemblance to bone (compare fig. 5). The true 
structure is, however, only seen in the cut and stained sections 
(see figs. 4, 6, 7, 8, 10, 11, and 12, together with their descrip- 
tion). The columns of soft cells above the papillse rise to the 
surface of the plates, and doubtless assist in promoting the 
wear of certain parts. Thus the papillse and columns are very 
minute or absent in the central ridge of the anterior plates 
(figs. 4 and 6), and in the raised border of the posterior plates 
(fig. 5). Conversely, the papillse, &c., are large and numerous 
in the concavities of all the plates, and they are seen in fig. 6 
to be especially large in the furrow of the anterior plate. The 
result of their presence is to render the corneous tissue friable, 
so that its surface becomes irregular as compared with that 
of the other parts. On examining the vertical sections it 
was found that the true surface was generally preserved where 
the papillae were small or absent, but that it was rarely intact 
over more than a very short length where these structures 
were large and numerous. Similar columns of soft cells, also 
rising from the apex of long narrow papillae, occur in other 
dense horny structures of Ornithorhynchus. Thus I have 
described them in the horny teeth of the tongue ( ‘ Quart. 
Journ. Micro. Sci./ vol. xxiii, N. S., PI. XXXII, fig. 7, l. s.). 
The deep surface of the posterior plates is in close proximity 
to the bone of their alveoli, being separated by a relatively 
thin layer of connective tissue representing mucosa, sub- 
mucosa, and periosteum (see fig. II). The character of the 
epithelial cells of various parts of the plates may be seen in 
figs. 8, 11, and 12 (see also description). In some sections of 
plates which had been softened in an alcoholic solution of 
VOL. XXIX, PART 1. NEW SER. C 
