28 
EDWARD B. POULIiON. 
portaut observation he is alluding to the posterior plates. He 
wrongly concluded that, in order to attain the adult form of 
plate, the animal must shed these structures. He found that each 
cheek-pouch in the female Platypus contained a “ concreted 
substance the size of a very small nut,” shown by the micro- 
scope to be made up of “ broken crystals.” The substance 
was evidently sand ; and I have found an even larger quantity 
in the pouches of a specimen kindly lent me by Professor 
Moseley. These observations bear in an important manner 
upon the wear to which the teeth must be subject. 
Heusinger ( ‘ Histologie,’ 1822) wrongly describes Home’s 
“fibres” as “hollow tubes,” evidently relying on ground 
sections of the dried plates, in which the papillae below and the 
soft cells above have dried up. This has been a most prolific 
source of error in the description of these structures, just as 
it was originally in the case of bone. 
Cuvier described the form and position of the horny plates. 
Sir Richard Owen ( f Odontography,’ 1840 — 1845, vol. i, pp. 
309—311) gives an historical account, to which I am indebted 
for reference to the authorities quoted above. He figures the 
position of the plates, and somewhat roughly indicates their 
shape. He describes the form of the teeth, but omits re- 
ference to the small third concave surface of the posterior 
plates in each jaw. He confirms Home’s important statement 
that each of the posterior plates is made up of two separate 
tubercles in the young animal. “ The subsequent conversion 
of this apparently double into a bituberculate single grinder is 
produced by the progressive extension and confluence of the 
bases of the tubercles, not by a process of shedding and the 
formation of a new tooth, as Home conjectured.” He 
wrongly supports Heusinger’s description of hollow tubes, and 
figures a horizontal (evidently dried and ground) section (vol. 
ii, pi. lxxvi, fig. 3), which is described as “ showing the 
concentric walls of the canals of the principal tubes, and the 
minute pores or cells of the denser cementing fibrous sub- 
stance.” The “ concentric walls ” are the epithelial cells con- 
centrically arranged round the column of soft cells above a 
