i.] ORNITHODELPHIA. 3 



can only be deduced from the two existing families, as 

 hitherto no extinct animals which can be referred to other 

 divisions of this remarkable and well-characterised group 

 have been discovered. These two isolated forms, in many 

 respects dissimilar, yet having numerous common characters, 

 which unite them together and distinguish them from the 

 rest of the Mammalia, are the Ornithorhynchida and the 

 Echidnid(* both restricted in their geographical range to the 

 Australian region of the globe. Taken altogether they 

 represent the lowest known type of evolution of the Mam- 

 malian class, and most of the characters in which they differ 

 from the other two sub-classes, 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 

 next group. The cerebral hemispheres, in Echidna at least, 

 are well developed and convoluted on the surface. The 

 auditory ossicles present a low grade of development, the 

 malleus being very large, the incus small, and the stapes 

 columelliform. They have no true teeth, though the jaws 

 of Ornithorhynchus are provided with horny productions, 

 which functionally supply their place. The coracoid bone 

 is complete and articulates with the sternum, and there is 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 other oviparous vertebrates separately into a cloacal 



B 2 



