GROUP A. 



IN this group may be provisionally associated the two sub- 

 classes Dipnoi and Coccosteomorphi. Some authors (A. S. 

 Woodward [503, 505], Eastman [128-9]) definitely place the extinct 

 Coccosteus and its allies in one sub -class with the Dipnoi ; this 

 step seems, however, to be premature, considering how incomplete 

 is our knowledge of the former, and how much the two groups 

 differ from each other. It still remains to be proved whether the 

 Coccosteomorphi were truly autostylic, and even then whether they 

 were more nearly related to the Dipnoi than to the Teleostomi. 

 The affinities of these sub -classes is discussed in greater detail 

 below. 



Sub-Class 1. DIPNOI. 



The Dipnoi are among the most interesting of fish. On the 

 one hand, they have a close affinity to the Osteolepidoti ; on the 

 other, they present many striking points of resemblance to the 

 Amphibia, which cannot all be put down to convergence. 



For the correct delimitation of this grpup we are chiefly 

 indebted to Huxley [230], Giinther [190J, and Traquair [447]. 

 Already in the Devonian epoch two strongly marked types of 

 structure are found among the Dipnoi, exemplified by Dipterus and 

 Phaneropleuron, showing that, even then, considerable divergence 

 had taken place. First founded by J. Midler for Lepidosiren, the 

 group of living Dipnoi was shown by Huxley to be characterised 

 by the autostylic attachment of the jaws. Subsequently Traquair 

 demonstrated that the fossil forms were likewise autostylic. 



The scales are cycloid and overlapping. In modern genera they 

 are thin, and embedded below the surface in pockets in the con- 

 nective tissue of the dermis (Fig. 200). Dipterus has thick scales, 

 with a shiny pitted surface. In microscopic structure they closely 

 resemble the scales of the Osteolepidoti, with an outer layer of 

 typical cosmine, separated by a trabecular layer of bone from an 

 underlying sheet of isopedine. Smaller elongated quadrangular 

 scales of the same structure extend in rows over the median and 

 paired fins to their very edge (Fig. 202). This scaling of the tins 

 is characteristic of the sub-class. 



230 



