TRUE TEETH AND HORNY PLATES OF ORN1THORHYNCHUS. 25 
Monotremes are the descendants of the ancestral Mammalia or 
not, it is quite certain that the higher mammals must at one 
time have passed through a condition such as now exists in 
the Monotremes, in nearly all parts of their organisation; and 
many powerful arguments can be brought against the as- 
sumption that the same stage has been reached independently, 
and at widely separated periods, in the course of organic evo- 
lution. Almost all recent work has strongly supported this 
argument, the only exception being Gegenbaur’s reseaches 
upon the mammary gland. I have already alluded to my own 
unpublished work upon the hairs of Ornithorhynchus, which 
will be found to enforce the argument in a most striking 
manner. It would, however, be inappropriate to give further 
supporting details on this occasion. It is sufficient for the 
purposes of the present paper to again point out that the 
presence of true mammalian teeth in Ornithorhynchus is, as 
far as it goes, evidence for the single origin of Mammalia, and 
against the theory suggested by Dr. Mivart. 
Professor II. G. Seeley ( f Proc. Roy. Soc.,’ vol. xliv, No. 267, 
p. 129) has suggested that the horny plates of the adult Oruitho- 
rhynchus are degenerate true teeth. This statement has ren- 
dered necessary the addition of a second part to the present paper, 
in which the structure of the horny plates is described in detail 
(see below). There is, however, one part of Professor Seeley’s 
paper which is better considered here. The writer enumerates 
various characters by which mammalian are usually distin- 
guished from reptilian teeth, and shows that there are many 
instances in which these characters fail. He applies this argu- 
ment to the horny plates and to the true teeth of Ornitho- 
rhynchus. I shall presently show that the former have struc- 
turally nothing whatever to do with true teeth, so that any 
argument based on such a supposition falls to the ground. I 
will now shortly prove that the true teeth exhibit all the typi- 
cally mammalian characters which could be possessed at the 
stage of development they have reached. Three of Professor 
Seeley’s characters — the presence of distinct sockets, the wear 
of the crown, and the method of replacement — cannot of course 
