CHAPTER V 
THE SUBCLASS PROTOTHERIA OR ORNITHODELPHIA 
General Characters The characters of the Prototheria can at 
present only be deduced from the two existing families, since 
hitherto no extinct animals which can be referred with certainty 
to other divisions of this remarkable and well-characterised group 
have been discovered. These two isolated forms, in many respects 
widely dissimilar, yet having numerous common characters which 
unite them together and distinguish them from the rest of the 
Mammalia, are the Ornithorhynchide and the Echidnide, both re- 
stricted in their geographical range to the Australian region of the 
globe. Taken altogether they represent the lowest type of evolution 
of the mammalian class, and most of the characters in which they 
differ from the other two subclasses tend to connect them with the 
inferior, vertebrates, the Sauropsida and Amphibia; for, though 
the name Ornithodelphia owes its origin to the resemblance of the 
structure of the female reproductive organs to those of birds, there 
is nothing especially bird-like about them. 
Their principal distinctive characters are these. The brain has 
a very large anterior commissure, and a very small corpus callosum, 
agreeing exactly in this respect with the Marsupials. The cerebral 
hemispheres, in Echidna at least, are well developed and convoluted 
on the surface. The auditory ossicles present a low grade of de- 
velopment, the malleus being very large, the incus small, and the 
stapes columelliform. The coracoid bone is complete, and articu- 
lates with the sternum, and there is a precoracoid (epicoracoid) in 
advance of the coracoid, while there is also a large “ interclavicle” 
or episternum in front of the sternum, and connecting it with the 
clavicles. There are also “epipubic” bones. The oviducts (not 
differentiated into uterine and Fallopian portions) are completely 
distinct, and open, as in oviparous vertebrates, separately into a 
cloacal chamber, and there is no distinct vagina. The testes of 
the male are abdominal in position throughout life, and the vasa 
