70 



PISCES. 



The earliest Crossopterygians, especially those with acutely- 

 lobate paired tins, exhibit a remarkable resemblance to the 

 Dipnoi. It is also noteworthy that their larger teeth are 

 sometimes characterized by a complexity of minute structure 

 (fig. 54), due to infoldings of the enamelled dentine wall, only 



FIG. 54. 



Transverse section of Holoptychian (Dendrodont) Tooth, much magnified. 

 Devonian; N. W. Kussia. (After Pander. ) 



equalled and exceeded by those of the great Labyrinthodont 

 Batrachia (fig. 90, p. 138). The hyostylic character of the skull, 

 however, is definitely proved in several genera (e.g., Holoptychius, 

 Rhizodopsis, and the Ccelacanthidse) ; while the arrangement of 

 the cranial roof-bones conforms to the normal Teleostomatous 

 type, and makes no approach to the anomalous condition 

 observable among Dipnoi. During the evolution of the Order, 

 (i.) the paired fins become abbreviated, (ii.) the supports of the 

 median fins tend towards reduction to a single series, (iii.) these 

 supports sometimes become correlated in part with the dermal 

 fin-rays, and (iv.) there is sometimes degeneration in the 

 external armour of the head and opercular region, some plates 

 being fused together, others being lost. The reduction in the 

 complexity of the mandibular ramus is especially noteworthy. 



