The Chondrostei and Holostei 337 



were the gymnospermic plant life and the reptilian life. In 

 part we believe that this is explained by the minor attention 

 given by palaeontologists to the regions involved, in part 

 to the greater difficulty attending correct identification of 

 vertebrate and not least fish remains. A very wide field 

 is here open for future study. 



Though our knowledge regarding intermediate and con- 

 necting stages in the history of the above four genera be- 

 tween Upper Liassic and commencing Cretaceous times, 

 is still sadly imperfect, it seems almost assured that species 

 of Hypsocormus gradually moved seaward, driven in part 

 probably by the abundant reptilian carnivorous forms that 

 preyed on them (p.217). In the process diverging struc- 

 tural details were acquired, that suggest analogous re- 

 semblances to some of the sharks and reptiles, that they 

 were contending against in the struggle for existence. 

 During the period therefore that intervened between Kim- 

 meridgian and early Cretaceous rock deposits, a new genus 

 evolved that shows close affinity with Hypsocormus, but 

 differs in the elongated snout, in the large sharp deeply- 

 set teeth, in the powerful pectoral fins, and the expanded 

 almost homocercal tail. 



This is the genus Protosphyraena, remains of which 

 are found in the Neocomian beds of Russia, in the over- 

 lying Gault and Upper Greensand of east central England, 

 but which became specially abundant and widespread 

 throughout the various "Chalk" strata from the Ceno- 

 manian and Turonian to the top of the Senonian formation, 

 and is included in Woodward's "Ganoid Fishes of the 

 Chalk" {2^8:2,02). It ultimately took possession of a 

 sea-territory that at least extended from Russia westward 

 across what is now S. England, to S. E. North America, 

 and thence onward to Kansas. In the latter region again, 

 it seems to have given rise to a related marine type, with 

 greatly elongated and flattened snout, that Cope has named 

 Erisichthe. 



So while the vast majority of protospondylian ganoids 

 continued — like their ancestors — to evolve in freshwater 

 territories, groups of the Pycnodonts and of the Pachycor- 

 mids passed almost simultaneously into the sea, during the 



