LIST OF ILLUSTRATIONS xxi 



FACING PAGE 



The Evolution of the Paddle of the Penguin 385 



After Pycraft. 

 Section of the Head of the Helmet Hornbill, showing the 



Dense Layer of Horn and Bony Tissue forming the Casque 422 



From a Specimen in the British Museum. 



The Trunk Skeletons of Chionis, the Diving Petrel and the 



Fork-tailed Petrel 451 



From Specimens in the British Museum of Natural History. 



The Trunk Skeletons of the Guillemot and Diver, showing 



Modifications Induced by Diving-habits 451 



From Specimens in the British Museum of Natural History. 



Shank of the Leg of the Red-throated Diver and of the Great 



Crested Grebe - - 452 



From Specimens in the British Museum of Natural History. 



