Ixii Introduction. 



stages of metamorpliosisj the development of limbs never exceeding 

 a pentadactyle division in their terminal segment, and possessing 

 the same segmentation as that seen in the higher Vertebrata, is to 

 be reckoned as an obvious external characteristic, which, together 

 with the absence of scales, diflPerentiates them, with very few ex- 

 ceptions, from Pisces. They never have median fin-rays supported by 

 dermal spines ; and, in the absence of an ossified basi-occipital, they 

 always have two condyles formed by the exoccipitals, for articu- 

 lation with the atlas. The heart has always two auricles, perfectly 

 separated ordinarily, and communicating with a single ventricular 

 cavity. 



Their integumentary system difiers from that of Fishes in not 

 having either dermal ossifications or dermal scales developed in 

 the region of the trunk ; Ceratophrys, however, and Brachycephalus 

 amongst existing Amphibia, form exceptions to this rule, having 

 dermal ossifications developed in their dorsal region; and the 

 CaecUiae develope dermal scales. In the Salamandra tmguiculata 

 again, and in the Bactyleilira capensis, a development of nails has 

 been observed, contrary to the rule that in the branchiate Verte- 

 brata there is no epidermal skeleton. The cutaneous system of 

 Amphibia {Triton) has been observed to possess, during their larval 

 life, rudimentary structures, resembling the sensory organs deve- 

 loped in Fish, in connection with the ' lateral line.^ In adult Am- 

 phibia, the cutaneous glandular system often attains a great deve- 

 lopment as in the ' parotoids '' and other glands of many Anura. 



The suspensorium is immovably articulated to the skull, and is 

 continuous with the pterygo-palatine elements of the maxillary 

 apparatus anteriorly, whilst externally it has applied to it a mem- 

 brane bone, homologous probably with the praeoperculum of Tele- 

 ostean Fishes. No Amphibian, however, ever possesses in the cu- 

 taneous opercular flap which it developes, any representatives of 

 the operculum, stib-operculum, inter-operculum, or branchiostegal 

 bones of Fish. The maxillary and praemaxillary bones are never 

 absent, and are ordinarily dentigerous. The vertebrae are very 

 numerous, and amphicoelian in the lower Amphibia; they are 

 few, and show, ordinarily, the procoelian, though, sometimes, the 

 opisthocoelian arrangement of the articvdar ends of their centra in 

 the higher orders. The neurocentral suture is usually absent. 

 Except in the serpentiform apodal CaecUiae, the ribs are rudi- 



