AMPHIBIA. 



175 



attached to the transverse processes. The bones of the fore- 

 arm {radius and ulna) on the one hand, and those of the 

 shank {tihia and fibula) on the other, are not anchylosed to 

 form single bones. 



Fig. 537.— Tailed Amphibians, a, Sirm lacertina ; b, Ampliiuma, showing the four minute 

 limbs ; c, Menobranchus maeulatus. (After Mivart. ) 



The best known of the existing Urodela are the Newts 

 {Triton), the Salamanders (Salamandra), the Mud - eels 

 (Siren), the Axolotl (Siredon), and the Giant -salamanders 

 {Menopoma). Some of these are " perennibranchiate," re- 

 taining the branchise throughout life ; others lose the 

 branchise, becoming thus " caducibranchiate," but retain the 

 branchial apertures behind the head ; others, lastly, lose 

 both the branchise and the branchial apertures. Most of 

 the Urodela have the four limbs well developed, but some 

 possess only the anterior limbs. 



The geological history of the Urodela is short, and of 

 comparatively little importance. The most ancient forms 

 which have been referred to this order are the Palmosiren of 

 Geinitz and the Protriton of Gaudry ; but the true position 

 of these is not altogether certain. The former is from the 

 Lower Permian, and is believed by its discoverer to be a 

 Urodelan, and to be most nearly allied to Siren lacertina ; 



