LECT. II.] EXTINCT BIRDS. 35 



these low beasts, or Prototheria, have a large median 

 clavicle (interclavicle) besides, and all the three bones are 

 simple, being merely ossified membranous tracts. 



So also are they in the Lizard tribe, and so were they 

 in those huge whale-like Lizards of the Secondary epoch 

 the Ichthyosauri whose triple clavicular structure is 

 much like that of the Monotremes. Birds, as a rule, 

 fuse these three bones together to make their merry- 

 thought or furcula, but in these there is a rudimentary 

 pro-coracoid cartilage, fused with the tops of the forks of 

 the merrythought, similar to the ordinary cartilaginous 

 nodule on the upper end of the mammalian clavicle. 

 The old tooth-bearing birds of the Chalk had their 

 clavicles distinct, as in the Emeu, and in embryo birds 

 generally. Here, again, to get at the root of the 

 Monotremes, we must dig below the bird, and if we are 

 safe in drawing any deductions whatever from our 

 morphological observations, we are safe in saying that 

 of a certainty the stock from which these beasts were 

 derived lay as low down as that from which the earliest 

 birds grew. 



Moreover, there is a sort of solid primitiveness about 

 the clavicles of the Monotremes, unlike what we see in 

 the existing Lizard, in which they are very slender and 

 graceful ; they show that the best type for comparison 

 is not the small modern Lizard, but the ancient reptilian 

 giant the Ichthyosaurus. 



The Prototheria have the sternum (or breast-bone) 



