122 



AMPHIBIA 



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 

 aixhes 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. 

 The Stegocephalian pectoral arch (Fig. 205) is of unique construction, 



differing from that of other amphibians in notable respects. The most 



characteristic, as well 



as the largest and 



usually best preserved 



bones, occupy an ex- 

 ternal position on the 



thorax between the 



pectoral limbs. These 



bones are three in 



number, and as a rule 



prominently sculp- 

 tured. The median 



element identified as 



the interclavicle (or 



oi-i + /-.ofovnnm\ -iroi^Too 'ff, Interclavicle ; cL .Clavicle ; co, Coracoid ; sc, Scapula [=cleithrum of 



entosteinum; vaiies oegenbaur]. 



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 



Cross-section of a tooth of Mastodoiisaunts jae.geri. 3/j (after Owen). 

 P, Pulp cavity ; c, Dentine tubules. 



Fig. 20u. 

 A, Pectoral arch of BranMosaiirus. B, Melanerpeton (after Credner). 



