52 



had a comparatively short range in time, remains of the group being 

 only found in the Silurian and Devonian rocks, thus it seems as 

 though unable to cojie 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 Flacoderms was generally car- 

 tilaginous, a condition considered by some authors as indicative 

 of a low organization, but as the quantity of bone composing their 

 external shields was much greater than that forming the internal 

 skeleton of the existing tyjjes of true bony fishes, and as traces of 

 ossified caudal vertebrae have been discovered in one genus they 

 ought rather to be highly plaed in a sj^stematic 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 li\nng plated Siluroid Teleosts of the African rivers. 



Distribution and Kange of the Piscine Families in 

 Geological Time. 



In considering the distribution and range of the various 

 famihes in a geological time, we find that authenticated remains 

 of sharks, Placoderins, and Cephalaspids have been obtained from 

 the Lower Ludlow Beds of the Upper Silmian rocks of Europe, 

 but in America it is singular that no fossil fishes haA'e as yet been 

 discovered before the Devonian epoch, when the relics of numerous 

 genera occur aliundantly, 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 deposition of the series. The Devonian formation 

 is built up of fresh-water, estuariue and marine strata, each group 

 characterised by its peculiar forais of Ufe. In the Old Red Sand- 

 stone of Scotland and Russia, fresh-water species in-edominate, 

 while in the marine limestones of Devonsliire, and the Eifel, 



