TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHUS. 27 
Multituberculata are Monotremata, and not Marsu- 
pialia, as has been supposed.” Professor H. F. Osborn has 
also written to me upon the subject, enclosing a proof plate of 
his ‘ Memoir upon the Mesozoic Mammalia,’ to be published 
shortly. The second and third teeth of Ornithorhynchus bear 
considerable resemblance to his figures of the second (lower) 
molar of Plagiaulax minor and of Ptilodus, except that the 
two chief cusps of these are on the internal border of the tooth 
instead of on the external border, as in the lower teeth of 
Ornithorhynchus. There is also some considerable resemblance 
to his figures of the teeth of Microlestes and of Bolodon. 
Certainly, as Professor Cope implies, the character of the teeth 
of Ornithorhynchus entirely negatives the argument that cer- 
tain secondary Mammalia must have been specialised rela- 
tively to the Monotremata, because of their multituberculate 
teeth. 
Part II. — The Horny Plates of Ornithorhynchus 
Paradoxus. 
Historical — Form aud Position of the llorny Plates — Structure of the 
Horny Plates — Probable Relation of the Posterior Horny Plates to 
the True Teeth. 
Historical. — Home (‘Phil. Trans.,’ 1802, p. 71) correctly 
describes these horny plates as differing “from common teeth 
very materially, having neither enamel nor bone, but being 
composed of a horny substance only embedded in the gum, to 
which they are connected by an irregular surface in place of 
fangs.” He describes the “ internal structure ” as “ fibrous, 
like nail ; the direction of the fibres is from the crown down- 
wards.” In this description he evidently alludes to the 
papillae and columns of softer cells above them, erroneously 
considering that they represent the hard part of the 
plate. He also says, “ In the smaller specimens before 
examined each of these large teeth appeared to be made up of 
two smaller ones, distinct from each other.” In this irn- 
