326 



AMPHIBIA 



xii. 13 



parietals, whereas the 'parietals' were the post parietals, which have 

 gone completely from most amphibians, though still present in the 

 earliest Devonian forms (Fig. 194). This is an excellent example of how 

 study of changes of proportion can clear up morphological difficulties. 

 The opercular apparatus covering the gills was lost early in 

 amphibian evolution; perhaps the reduction of the whole posterior 

 part of the head was effected by a single morphogenetic change. In 



ppa. "— — 



Osteo/epis 



F/p/ftOStCLJC 



Ichthyostega 



sub 



Palaeogurinus RomerJa Dimetrodon 



(Cotylosaur) (Pelycosaur) 



Fig. 194. Skulls of a crossopterygian and various early tetrapods to show the 

 shortening of the posterior region. Lettering as Fig. 193. (After Westoll.) 



modern amphibia the skull is much flattened and its ossification 

 reduced, so that large spaces are left; in the earlier forms, however, 

 the skull was of the more usual domed shape and the roof and jaws 

 were covered by a complete set of dermal bones. Presumably the loss 

 of bone was another development producing a reduction of weight 

 advantageous to a terrestrial animal. 



Lateral line organs are present in aquatic amphibians and their 

 position is marked on the bones of the fossil skulls by rows of pits. 

 By using these lines as reference marks it is possible to compare the 



