54 ANNUAL REPORT OF 



Concerning the origin of Lung-fishes as a group, it is further 

 observed in the same work, that "it seems reasonable to look for 

 their ancestors in the early Devonian Crossoptergii with acutely 

 lobate fins, or, with greater probability, to> some still more primi- 

 tive Crossopterygian with simple, non-rhizodont teeth, capable by 

 fusion of giving rise tO' massive tritoral plates." Throughout the 

 Palaeozoic this group of fishes formed a conspicuous feature of 

 the vertebrate fauna, and although they appear to have attained 

 their maximum development and specialization during the Devon- 

 ian, they did not become entirely insignificant until near the close 

 of the Trias. Teeth similar to* those of the recent Neoceratodus 

 are profusely distributed throughout Triassic rocks of the world, 

 but none have been obtained thus far from the eastern part of the 

 United States. It has been shown in a highly instructive manner 

 by Dollo that the oldest known Lung-fishes, such as Dipterus 

 (Fig. 7) and its associates, are the most archaic, and that 

 their modern representatives have been derived from them by a 

 series of retrogressive changes; or, in other words, the latter 

 have much the same relation to the former as the degenerate stur- 

 geons and paddle-fishes to their Palaeozoic ancestors, the Palceon- 

 iscidce. Owing to a fortunate abundance of material it is possible 

 to select a series of genera, beginning with Dipterus, and termin- 

 ating with modern forms, which illustrate the evolution of the 

 group both in structure and in chronological sequence. Some 

 of the more important structural modifications observable in the 

 transition from the older to the recent genera are : ( 1 ) the grad- 

 ual union of isolated median fins to form a continuous fin; (2) 

 the conversion of a heterocercal tail into a symmetrically formed 

 one; (3) the degeneration of the squamation, the thick, ganoid 

 scales of the earlier types being replaced by thin, non-ganoid 

 scales, or the skin becoming almost entirely naked; (4) a reduc- 

 tion in the number of cranial dermal bones and the loss of their 

 original ganoid investment; (5) the suppression of the jugular 

 plates, and (6) a reduction in the size of the opercular bones. 



Ganoids. — The final and one of the most important contin- 

 gent of the Devonian fauna is furnished by Ganoids, or enameled- 

 scale fishes. These are divided into two orders, the so-called 

 "fringed-fmned" {Crossopterygian) and the "stout-finned" {Ac- 



