Related Geological Conditions 23 



CHAPTER II 



Geological Conditions in Relation to the 

 Evolution of Fishes 



The first clear evidence we have of fossil fish remains 

 is in rocks of upper Silurian age. The celebrated fish- 

 beds or bone-beds of the Downton series in England, of the 

 Kincardine series in Scotland, of the Waterlime in North 

 America, and of the Oesel beds in Russia, suddenly reveal 

 a varied and bizarre variety of fishes, of which we now 

 have no direct living representatives. Such occurrences 

 prove that long anterior to the time of deposition of 

 these beds, still more ancient and ancestral forms of fish 

 had evolved from some phylum or phyla of the inverte- 

 brates. This must have occurred during Ordovician, 

 or even during Cambrian time. 



So in chapter 3 the writer extends his previously pub- 

 lished proof that the Nemertinea is the invertebrate group 

 from which all of the chordate or notochordal divisions 

 have started. As existing at the present day the number 

 of nemertean genera is forty-two and of species fully four 

 hundred. But since all of those now existing have soft 

 and readily decomposable bodies, they would leave no fos- 

 silized traces in the rocks, or might at most and under 

 rare conditions have their firmer tissues indicated. For 

 the occasional preservation of medusoid, of trilobite, and 

 even of soft fish tissues has already been demonstrated. 

 Similar nemertean remains, however, are still unknown, 

 unless indeed some of the "conodont" genera are the teeth 

 of nemerteans or of primitive cyclostome fishes. 



A few amongst other reasons for regarding the Nemer- 

 teans as being an ancient group geologically are: — (a) 

 representative living species occur in practically every part 

 of the world, and this suggests a slow but extensive dis- 

 tribution, and gradual adaptation to environal surround- 

 ings, of existing genera; (b) two genera and fifteen species 

 are either freshwater or terrestrial in habitat, though the 

 majority are now marine littoral dwellers, while two genera, 

 Pelagonemertes and Malacohdella are pelagic; (c) even 



