TRUE TEETH AND HORNY PLATES OE ORNITHORHYNCHUS. 29 
papilla; the latter cells, dried up, constitute the “ canals of the 
principal tubes,” while the " minute pores or cells ” are the 
dried-up softer centres of the corneous epithelial cells which 
make up most of the horny plates. 
Professor H. G. Seeley, as I have already mentioned, con- 
siders that the horny plates are degenerate true teeth. He 
considers that each of the posterior plates consists of "three 
teeth on each side closely united together into one long ovate 
mass.” Sir Richard Owen quotes a French analysis showing 
that the plates consist almost entirely of horny substance. 
Professor Seeley considers that this " can hardly apply to 
the posterior teeth,” but he gives no evidence in support of 
such an opinion. He observes that the central concavities of 
each of the posterior plates is opaque, while the peripheral 
border is translucent and horny. This appearance is merely 
due to the fact that the former contains abundant papillae and 
columns of soft cells, which dry up and contain air, while 
these structures are only faintly represented in the latter 
locality. But the author suggests that the border represents 
“ the uncalcified enamel of the tooth, while the central portion 
corresponds to the dentine.” He gives no evidence, histo- 
logical or developmental, for the actual occurrence of a struc- 
tural change so unique as the conversion of uncalcified enamel 
into dense, translucent horn. Furthermore, I have already 
shown that the enamel of the true teeth is calcified and hard, 
and that it has reached a condition which a widespread ex- 
perience of vertebrate dental tissues proves to be the climax of 
histological differentiation. Professor Seeley supports his 
identification of the central parts of the plates with dentine, 
by describing certain appearances seen in sections which he 
interprets as due to the presence of bony tissue. If the inter- 
pretation were correct it would hardly support the writer’s 
conclusions, for I have shown that the dentine of the true 
teeth is as typical as their enamel, and has reached a stage of 
differentiation at which its conversion into bone would be as 
improbable as the change of enamel into horn. 
But Professor Seeley’s statement that bony tissue is present 
