CHAPTEE 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 Ornithorhynchidce and the Echidnidce, 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 



