12-2 



AMPHIBIA 



CLASS II 



tubules. This gives rise to the highly characteristic " labyrinthodont " 

 structure, which reaches its extreme development among the larger and 



geologically later Stego- 

 cephalians (Fig. 204). The 

 internal structure becomes 

 progressively less compli- 

 cated toward the apex of 

 the tooth, above the zone 

 where radial infoldings of 

 the walls leave off, the 

 dentine is traversed simply 

 by straight tubules radiat- 

 ing from the pulp cavity. 

 The insertion of the teeth 

 may be of the acrodont or 

 pleurodont type, as already 

 described, or they may be 

 seated in shallow alveoli. 



Traces of branchial 

 arches being conspicuous 

 in the young of several 

 Palaeozoic genera, the sup- 

 position is natural that 

 respiration was by gills 

 during the early stages of 

 all Stegocephalians. 

 is of unique construction, 



The most 



Fig. 204. 



Cross-section of a tooth of Mastodonsaurus jaegeri. % (after Owen). 

 P, Pulp cavity ; c, Dentine tubules. 



The Stegocephalian pectoral arch (Fig. 205) 

 differing from that of other amphibians in notable respects 

 characteristic, as well 

 as the largest and 

 usually best preserved 

 bones, occupy an ex- 

 ternal position on the 

 thorax between the 

 pectoral limbs. These 

 b@nes are three in 

 number, and as a rule 

 prominently sculp- 

 tured. The median 

 element identified as 

 the interclavicle (or 

 entosternum) varies 

 considerably in form 

 and size amongst different genera. Most frequently it is rhombic, but 

 sometimes transversely oval, and may terminate behind in a long median 

 extension. 



Partly overlapping the forward portion of the interclavicle are the paired 

 plates which correspond to the clavicles of other amphibians. Their posterior 

 ends are slender and slightly curved upwards ; the anterior end is expanded 

 into a triangular or oval plate, often sculptured, but sometimes smooth as if it 



Fig. 205. 



A, Pectoral arch of BrancMosaurus. />', Melanerpeton (after Credner). 

 id, Interclavicle; d, Clavicle ; co, Coracoid ; sc, Scapula [ = cleithrum of 

 Gegenbaur]. 



