Vv EGGS OF PLATYPUS LOY, 
of eggs, or at least ovoviviparity, would follow from the struc- 
ture of the egg, since the abundance of yolk would do away 
with the necessity for a placenta. That the eggs had this 
Saurian characteristic was first definitely made known by Pro- 
fessor Poulton! for Ornithorhynchus, and his results were con- 
firmed later for Hehidna” The structure of the eggs has, 
however, already been dealt with on p. 72. The fact that 
these animals lay eggs appears to have been known for a very 
long time, though rediscovered so lately as 1884 by Mr. 
Caldwell? In connexion with the structure of the ova, the 
ovaries themselves and the oviducts are built upon the Saurop- 
sidan plan. In the male the testes retain the primitive ab-/ 
dominal position. The fact that the urimary and _ genital 
products escape by means of their ducts into a chamber which 
also receives the end of the alimentary tract is not a distinctive 
feature of this group, inasmuch as it is seen in the Marsupials, 
and also in certain low Eutheria, such as the Beaver and other 
Rodents, and a few Insectivores. As to external features, the 
Monotremata show certain archaic characters. The unspecialised 
arrangement of the mammary glands has already been described. 
These animals are plantigrade, if the term may be used also to 
describe the aquatic Ornithorhynchus. The ears are absolutely 
destitute of a conch. The remarkable spur upon the hind-legs 
furnished with a gland, which is more marked in the male, and 
indeed disappears in the female of Ornithorhynchus, is a structure 
which argues the specialised condition of these two modern 
representatives of what must have been a large order in the past. 
The skeleton shows numerous ancient characteristics. In 
the skull there is no demarcation of the orbit from the temporal 
fossa, a feature widely found in archaic mammals. The tympanic 
remains as a slender ring, there being no auditory bulla formed 
either from this or from any other bone. The malleus and 
incus are large, and thus reminiscent of the quadrate and 
articular bone of reptiles. In the lower jaw the absence of 
a marked coronoid process, and the absence of a firm ossification 
at the meeting of the two rami, may be a primitive state of 
1 Quart. Journ. Mier. Sci. xxiv. 1884, p. 124. 
* Beddard, Proc. Roy. Phys. Soc. Edinb. viii. 1885, p. 354. 
3 See Phil. Trans. elxxviii. 1887, where the literature of the subject is fully 
cited. 
