190 THE VERTEBRATE SKELETON. 



auditory apparatus, forming the auditory ossicles 1 . Each 

 ramus of the mandible always consists of a cartilage bone, 

 the articular, and several membrane bones. The mandible 

 articulates with the cranium by means of a quadrate. The 

 ribs in Birds and some Reptiles bear uncinate processes, i.e. 

 small, flat, bony or cartilaginous plates projecting backwards 

 from their posterior borders. The sternum is not transversely 

 segmented as in mammals, and there are commonly distinct 

 cervical ribs. The ankle joint is intertarsal, or situated 

 between the proximal and distal row of tarsal bones, not 

 crurotarsal as in Mammalia. 



CLASS I. 



The axial skeleton is generally long, and that of the limbs 

 frequently comparatively short, or sometimes absent. 



The exoskeleton generally has the form of epidermal 

 scales, which are often combined with underlying bony dermal 

 plates or scutes and may sometimes form a continuous armour. 

 Neither feathers nor true hairs are ever present. The verte- 

 bral column is generally divisible into the five usual regions. 

 The centra of the vertebrae vary enormously, and may be 

 amphicoelous, procoelous, opisthocoelous or flat, but they never 

 have saddle-shaped articulating surfaces. The quadrate is 

 always large, and is sometimes fixed, sometimes movable. 

 A transpalatine bone uniting the pterygoid and maxilla is 

 generally present. 



Free ribs are often borne along almost the whole length 

 of the trunk and tail, and often occur attached to the cervical 

 vertebrae. The sacrum is generally composed of two vertebrae 

 which are united with the ilia by means of expanded ribs. 

 The sternum is rhomboidal, and may either be cartilaginous 



1 H. Gadow, Phil. Trans., vol. 179, 1888. 



2 See G. Baur, J. Morph., vol. i., 1887. K. Lydekker, Catalogue of 

 the Fossil Reptilia and Amphibia in the British Museum, Parts i. & u. 

 C. K. Hoffmann, Reptilien, in Bronn's Classen und Ordnungen des Thier- 

 reichs, Bd. vi., 3 abth. 187990. 



