314 



CLASS AMPHIBIA. 



two-headed are short and never completely encircle the thorax so as to 

 reach any sternal elements. The skull is in many respects amphibian- 

 like but in some features it recalls the Reptilia. It is always covered by 

 a number of bony plates which appear to have been dermal in position 

 and recall in many respects the condition found in ganoid fishes and in 

 crocodiles. In many forms the occipital region of the cartilaginous 

 cranium appears to have been unossified, and there is a ring of small 

 sclerotic plates round the eye. In some of the palaeozoic genera 

 (Branchiosaurus etc.) unmistakable remains of bony branchial arches are 

 present. The teeth are conical ; they are usually present on the jaws 

 and sometimes on the palatines and vomers. In the Labyrinthodonts 

 the pulp-cavity gives off branching diverticula into the dentine and the 

 surface of the dentine is folded in a complicated manner as in certain 

 extinct crossopterygian fishes. The shoulder girdle, though imperfectly 



known appears to have 

 had scapula (cleith- 

 rum), clavicle, coracoid 

 and interclavicles. The 

 limbs are on the normal 

 pentadactyle type and 

 present no approxima- 

 tion to those of fishes. 

 The hand when known 

 presents 4 digits, the 

 foot 5. 



FIG. 176. Branchiosaurus amblystomus Credner. 

 A skeleton of adult (nat. size). B restoration 

 of a larva with branchial arches (after Credner, 

 from Zittel). 



The Stegocephali 

 appear to have been 

 fresh-water or ter- 

 restrial animals. 

 Although generally 

 referred to the 

 Amphibia, their 

 systematic position 

 cannot be regarded 

 as fixed. The reason 

 for regarding them as Amphibia is the presence of gill- arches in 

 some genera, the fact that the ribs never so far as is known 

 reach a sternum and that traces of the lateral line system of 

 sense-organs are suggested by grooves on some of the cephalic 

 scales. But it must be remembered that these and other features 

 often asserted as characteristic of the Stegocephali have only 

 been found in some forms ; and that some members of the group 

 possess features which suggest reptilian affinities. We shall 

 probably not be far wrong in assuming that the order as at 

 present constituted is a composite one, containing genera some- 



