1898] IMPERFECTION OF THE GEOLOGICAL RECORD 329 



lake in the Devonian or Old lied Sandstone period, and have been 

 worked for commercial purposes from time immemorial. Fossil 

 fishes have been known and collected from them for more than 

 seventy years. Every exposure has been searched by expert collec- 

 tors, whether in the cliffs or in quarries. Yet, Palaeospondylus has 

 only been found in one very thin stratum in a single quarry, where 

 it occurs, not as a rarity, but in countless numbers. It seems as if 

 a shoal of the species had been accidentally destroyed and suddenly 

 covered up ; and it is a fortunate accident that a small quarry has 

 been opened at the precise spot. 



Our knowledge of the earliest shark-like fishes exhibiting the 

 most primitive type of paired fins is almost equally scanty. 

 CladoselacKe, in a state fit for accurate scientific study, has hitherto 

 been met with only in a flagstone at the base of the Carboniferous 

 formations in Ohio, U.S.A. Teeth of the same kind have been 

 known for many years from several parts of the northern hemi- 

 sphere ; but the complete fish has only been discovered in Ohio 

 within the last decade, and even now the skeleton of the head and 

 vertebral axis remains practically unknown. 



In some instances the old Palaeozoic forms of fish-life which 

 withdrew to the comparatively peaceful realms of rivers and fresh- 

 water lakes after the vigorous period of their race was past, and 

 survived until the present day, were thus entirely lost to geological 

 records. For example, three detached teeth from the English 

 oolites and scarcely more from corresponding rocks in Colorado, are 

 the sole known traces of the Dipnoan fishes between their world- 

 wide distribution at the dawn of the Mesozoic era and the scattered 

 remnants which still survive in the fresh-waters of South America, 

 Africa, and Queensland. Similarly, there is no doubt that Pulyptcrus 

 and Calamoichthys existing in the fresh-waters of tropical Africa, 

 are the direct and little-altered descendants of some of the Palaeozoic 

 fringe-finned ganoids ; but we have still not found even a trace of 

 them in the Mesozoic or Tertiary strata in any part of the world. 

 They must have lived somewhere, but the geological record, so far 

 as explored, is too imperfect to afford a clue to their whereabouts 

 and history. 



The case of the Amphibia or Batrachia is still more remarkable ; 

 though perhaps they, too, have been fresh- water animals since the 

 Palaeozoic era. It is definitely proved that some of the early lung- 

 breathers belonged to this class ; for traces of gill-arches are occasion- 

 ally observed in young individuals, showing that they breathed by 

 gills in their immature state (Branchiosaurus, Archegosaurus). It is 

 also certain that these primitive Amphibia were the dominant type 

 of vertebrate life from their appearance until the middle part of the 

 Permian period. In early Mesozoic times, however, they suddenly 



