50 ANNUAL REPORT OF 



extensive ; but with the close of Palaeozoic time the family became 

 decadent. 1 The group of Elasmobranchs as a whole, however, 

 began to flourish anew during the Mesozoic, gradually acquiring 

 fresh evolutional vigor. It cannot be said to show signs of 

 decadence in modern times, since it is represented in apparently 

 undiminished numbers in the marine fauna of the present day. 

 No members of the group have yet been discovered in the New- 

 ark rocks of New Jersey or New England. 



Dipnoans. — The Devonian, which has justly been styled the 

 "age of fishes," is remarkable for the introduction of two great 

 subdivisions of Pisces, known commonly as Dipnoans and 

 Ganoids, both of which are represented sparsely in the modern 

 fauna. To* the Dipnoans, or Lung-fishes, belong the "Barra- 

 munda" or Neoceratodus of Queensland, and two other genera, 

 one of them (Protopterus) inhabiting African, and the other 

 (Lepidosiren) South American rivers. As indicated by the com- 

 mon name of Lung-fishes, or Dipneusti, these fishes are remark- 

 able for having pulmonary respiration, there being a functional 

 lung (paired in the two last-named genera) in addition to' the 

 regular gills, thus enabling them to live out of water for con- 

 siderable periods. The action of the lungs in conjunction with 

 the gills furnishes a suggestion as to> the manner in which air- 

 breathing vertebrates have probably originated from gill-breath- 

 ing, fish-like progenitors. Indeed, owing to the striking resem- 

 blances which Dipnoans present to amphibians in their vascular 

 system and lungs, many have supposed that the former group 

 was directly ancestral to the latter. The best modern opinion, 

 however, is inclined to doubt that these resemblances are indica- 

 tive of direct ancestral relations, regarding them rather as the 

 outcome of physiological convergence, associated with adaptive 

 and parallel modifications in structure, and due to the influence 

 of a similar environment. It appears more probable that both 

 Lung-fishes and amphibians have been derived from some primi- 

 tive crossopterygian (ganoid) ancestor, not very divergent from 

 the Elasmobranch stem, and subsequently became modified in 

 certain respects along parallel lines. 



1 Hay, O. P., The Chronological Distribution of the Elasmobranchs. Trans. 

 Amer. Phil. Soc, vol. xx. (igoi), pp. 63-75. 



