350 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



only, as in the rabbit, and still more in the dog, or 

 absent, as in bears and all Ungulates. The iiiter- 

 clavicle is developed only in Prototheria. 



In the Teleostei the pubic arch may be placed far 

 forwards, and be thoracic, when the hind-fins lie 

 below the fore-fins ; or jugular, when they lie in 

 front of them (Fig. 134). In the higher forms they 

 always retain their position between the abdomen and 

 the tail. In recent reptiles the ilium may extend 

 far back, while in birds, and in certain extinct 

 reptiles (Dinosaurs) it is developed anteriorly to the 

 acetabulum, or cavity of articulation for the head of 

 the hind limb. In the Sauropsida this cavity is never 

 completely bony, and the same is the case in Echidna 

 (though not in Ormthorhynchus) ; with this exception 

 the acetabulum is always a completely bony cup in 

 mammals. 



In correlation with the posture or mode of pro- 

 gression, the ilium enters into more or less close union 

 with the sacral region of the vertebral column, and 

 the demands made upon the axis for further support 

 are responded to by the fusion of presacral or post- 

 sacral (or both) vertebrae with those of the true 

 sacrum to form a solid piece. Thus, in a bird 

 (Fig. 147) the whole arch is of great size, while in the 

 Cetacea it is at most represented by the ischia. 



Just as there are very many striking and sugges- 

 tive points of resemblance between the fore and hind 

 arches, so are the fore and hind limbs arranged on 

 essentially similar principles. 



It will be most convenient to begin with what 

 obtains in the Amphibia and Amniota, or the peiita- 

 dactyle Vertebrata. Either limb may be divided into 

 three regions : (a) arm, fore-arm, hand ; () thigh, leg, 

 and foot. In the arm, and in the thigh, there is a 

 single bone : (a) Iiumerus, or () femur; in the 

 fore-arm and leg two : (a) radius and ulna ; 



