﻿214 Miss Agnes Crane — On Certain Living and Fossil Fishes. 



in. the Upper Silurian and Devonian rocks ; thus it seems as thongli, 

 unable to cope in the struggle for existence with the lighter-armed 

 and more active race of ganoids which predominated in the Devonian 

 waters, they died out, leaving no immediate descendants. The 

 vertebral column in the Placoderms was generally cartilaginous, a 

 condition considered by some authors as indicative of a low organi- 

 zation, but as the quantity of bone composing their external shields 

 yvas much greater than that forming the internal skeleton of the 

 existing types of true bony fishes, and as traces of ossified caudal 

 vertebrge have been discovered in one genus, they ought rather to be 

 highly placed in a systematic classification. The group is considered 

 by Professor Huxley to form a link between the Ganoids and the 

 Teleosts, and as having most affinity with the living plated Siluroid 

 Teleosts of the African rivers. 



Distribution and Eange of the Piscine Families in Geo- 

 logical Time. — In considering the distribution and range of the 

 various families in geological time, we find that authenticated remains 

 of sharks, Placoderms, and Cephalaspids have been obtained from the 

 Lower Ludlow Beds of the Upper Silurian in Europe, but in 

 America it is singular that no fossil fishes have as yet been discovered 

 before the Devonian epoch, when the relics of numerous genera occur 

 abundantly, differing, however, from the European forms. This 

 dissimilarity in the fauna is probably owing to the differences 

 existing in the physical geography of the two areas at the time 

 of the dep)Osition of the series. The Devonian formation is built up 

 of freshwater, estuarine and marine strata, each group characterized 

 by its peculiar forms of life. In the Old Red Sandstone of Scotland 

 and Russia, freshwater species predominate, while in the marine 

 limestones of Devonshire, and the Eifel, Mollusca, Corals, and the 

 remains of genera of inshore dwelling fish indicate a shallower 

 marine deposit. The greater part of the American Devonian, on the 

 contrary, was apparently laid down in an open sea, and thus a 

 monster marine fauna flourished, not so generally represented in 

 Europe ; but it is interesting to note the identity of a few species 

 occurring in localities where the beds are of similar structure to 

 those of contemporaneous age in Europe. In both worlds the for- 

 mation is alike distinguished by the great preponderance of ganoid 

 over elasmobranchiate fishes. The conditions existing during the 

 formation of the Devonian rocks are well illustrated at the present 

 day by the freshwater lakes, mighty rivers, and extended coast-line 

 of the African and American continents, and it is a most suggestive 

 and significant fact that the genera of living ganoid and dipnoid 

 fishes most resembling the Paheozoic forms are now, with two 

 exceptions, found on those continents alone. Taking the various 

 orders of Professor Huxley's comprehensive classification in succes- 

 sion, we find that no traces of the first or lowest order, the Pharyn- 

 gobranchii, which contains only the " gullet-breathing " Lancelet, 

 have been found in a fossil state. This is easily accounted for, 

 however, by the soft and perishable structure of the species, of 

 which no remains could possibly be preserved in the finest sedi- 



