Distribution of Primitive Fishes 483 



CHAPTER XVII. 



The Geographic and Geologic Relations of 

 THE More Primitive Fishes. 



As passage backward is made to the older rock-strata, 

 and as types of fish are encountered that depart more and 

 more in species, genera, and families from the abundant 

 ones of recent times, it becomes correspondingly difficult 

 to trace the extent and trend of the land-masses over which 

 freshwater areas existed, that enabled migration of fresh- 

 water fishes to proceed. So there is equal difficulty in trac- 

 ing the exact distributional lines followed by these fishes. 

 Further, we still lack definite information as to whether 

 certain strata and their fish-remains were of freshwater or 

 of marine origin. For as has often been pointed out in 

 previous chapters, vast deposits that were assigned to the 

 latter, should by clearest evidence be grouped under the 

 former or freshwater mode of deposit. 



Having already dealt with the relation of the Teleosts 

 to past land-masses, we can now study those transition fami- 

 lies of fishes that connect the Teleosts with the cartilagin- 

 ous fishes, viz. the Leptolepidae, Oligopleuridae, Archa- 

 eomenidae, and Pholidophoridae. Along with these the 

 more primitive Holostei or Protospondylii, and the Chon- 

 drostei can be linked up in their land connections. 



From statements already made (pp. 343-345) it would 

 appear that during early Cretaceous and much of Jurassic 

 time, an extensive and more or less continuous freshwater 

 passageway existed between central and south Europe, 

 Central Siberia, China and Australia. For Lycoptera, 

 Leptolepis, Oligopleurus, Oenoscopus, Archaeomene, 

 PleurophoUs and other related types extend over this en- 

 tire region either side by side, or so dovetailing into each 

 other from one section of the above-named land-masses 

 to another, that geographic and geologic continuity become 

 a necessity. Thus Leptolepis^ as already shortly referred 

 to (p. p. 206-207) includes species from Western England, 

 Central and S. Europe, and from the Talbragar beds of 



