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THE AMERICAN NATURALIST [Vol. XLIX 



the Devonian age, — an epoch in which, so far as onr present knowledge 

 goes, no fish belonging to the suborders Amiadse or Lepidosteidae (unless 

 Cheirolepis is one of the latter) makes its appearance. Rapidly dimin- 

 ishing in numbers the Crossopterygidae seem to have had several repre- 

 sentatives during the Carboniferous epoch, but after this period . . . 

 they are continued through the Mesozoic age only by a thin, though 

 continuous line of Ccelacanthini, and terminate, at the present day, in 

 the tAvo or three known species of the single genus Pohjpterus, now 

 recognized under two genera: Polypterus and Erpctoichtlujs (Cahttn- 

 oichthys). 



Such, in brief, is the geological history of this peculiar 

 order of fishes. At the time of the publication of Hux- 

 ley's essay the fact of the geological distribution of the 

 group was of the greatest interest, since it proved the 

 transition of a vertebrate group from the Paleozoic to 

 later times in practically unchanged form. 



It will thus be seen that in the Devonian we have repre- 

 sentatives of these two groups; one of which has been 

 supposed to have given origin to the other. The oldest 

 known Crossopterygian coexisted, in the Devonian, with 

 well-developed Amphibia-like forms if we may trust the 

 evidence of the single imprint of Thinopus antiquus 

 Marsh from the Devonian of Pennsylvania. That the 

 impression described by Professor Marsh is in reality a 

 footprint no one, who consults his figures, can doubt. 

 Its geological horizon is vouched for by the late Dr. 

 Charles E. Beecher, whose interpretations of geological 

 facts have never been impeached. So that we may say 

 with perfect assurance that the oldest known Crossop- 

 terygia existed side by side, geologically, frith well-devel- 

 oped air-breath in g, quadrupedal vertebrates. Whether 

 the latter were Amphibia or not is a matter which no one, 

 in the light of our present knowledge, can decide. The 

 inference, however, is that they were such. 



In order that all the facts of the case in regard to the 

 origin of the Amphibia may be given, the following defi- 

 nition of Crossopterygia is given. It is based on the 

 first definition by Huxley and the definition contained in 

 Zittel's "Paleontology" (ed. by Eastman). 



Dorsal fins two. or if single multifid and very long; the pectoral and 



