THE ORDERS OF THE AMPHIBIA. Ill 



of the cervical vertebrae are sometimes concave behind and con- 

 vex in front (opisihocoelous). There are four, or more, vertebrae 

 in the sacrum. 



The pelvis and bones of the hind limb are in many respects 

 very like those of birds. No clavicles have been observed, and 

 the fore limb is sometimes very small in proportion to the hind 

 limb.* 



III. THE ICHTHYOPSIDA. 



The AMPHIBIA are divisible into four orders 



1. The UEODELA. There is no exoskeleton. The dorsal 

 vertebras are amphiccelous or opisthocoelous, and have single or 

 bifid transverse processes, to which short ribs of a corresponding 

 form are attached. The single sacral vertebra has movable 

 ribs like the rest, when a sacrum exists. The caudal vertebras 

 are numerous and distinct. The bones of the fore-arm and of 

 the leg remain separate, and the proximal bones of the tarsus 

 are not elongated. 



This order comprises the Newts and Salamanders, with the 

 so-called " perennibranchiate " Amphibia, such as the Proteus, 

 Siren, &c. 



2. The BATRACHIA. An exoskeleton is rarely represented 

 by dermal ossifications in the dorsal region of the trunk. 

 The dorsal vertebrae are proccelous, and have simple and 

 long transverse processes, with only rudimentary ribs. The 

 single sacral vertebra has wide lateral processes for articula- 

 tion with the ilia, and no movable ribs. A styliform ossifi- 

 cation takes the place of the centra of the caudal vertebrae. 



The ischia and pubes of opposite sides are applied together, 

 and unite by their inner faces. The radius and ulna in the 

 fore limb, and the tibia and fibula in the hinder extremity, unite 

 into a single bone. The calcaneum and astragalus are greatly 

 elongated. 



The Frogs and Toads compose this order. 



* In Iguanodon, Megalosaurus, PoiMlopleuron, and Scelidosaurus, the distal 

 end of the tibia extends outwards behind the fibula in a manner which is ex- 

 tremely peculiar and characteristic ; and the astragalus is very like that of 

 a bird. 



