30 
EDWARD B. POULTON. 
in the central parts of the horny plates is only an extreme form 
of the error which has followed from the exclusive examination 
of dried and ground -down sections. In PI. IV, fig. 9, I 
have shown the appearance of a horizontal section prepared in 
this way, and it will be seen that there is some superficial resem- 
blance to bone. Sir Richard Owen’s “canals of the principal 
tubes” are Professor Seeley’s “large Haversian canals,” while 
the “ minute pores or cells ” correspond to the “ canaliculi ” 
described by Professor Seeley, which radiate from apparent 
lacunae. If the more highly magnified fig. 8 be compared with 
fig. 9, it will be at once seen that the deceptive resemblance to 
bone is entirely due to the presence of air in the spaces left 
by the dried-up softer parts, and it will be clear that the whole 
tissue is typically epithelial. Different sections will include 
more or less air according to slight differences in preparation, 
and hence the supposed bone could only be detected “ in some 
specimens.” Rut Professor Seeley can only have looked at 
horizontal sections of the plates, for the first glance at a vertical 
section, however prepared (see figs. 4 and 5), would prove that 
the resemblance to bone was merely delusive. Since the above 
was written I have been enabled, through the kindness of 
Professor Stuart, to examine the sections in the Museum of the 
Royal College of Surgeons, upon which Professor Seeley bases his 
opinion. They entirely confirm the interpretation at which I 
had previously arrived, and of which an account is given 
above. There are several dried and ground-down sections, 
containing different amounts of air, and those which contain 
most air are fairly represented by fig. 9. Among them are 
vertical and oblique sections and horizontal sections with the 
air displaced. It is difficult to understand how Professor 
Seeley can have reconciled the appearance of these latter with 
his interpretation of those horizontal sections which still re- 
tain a considerable quantity of air. 
Professor Seeley’s suggestion that the true teeth may, 
perhaps, be looked upon as successional to the horny plates 
may be at once dismissed, for the former not only appear long 
before the latter can be identified with any certainty, but have 
