CHAPTER XV. 



DISTEIBUTION OF FISHES IN TIME. 



Of what kind the fishes were which were the first to make 

 their appearance on the globe ; whether or not they were 

 identical with, or similar to, any of the principal typos exist- 

 ing at present ; are questions which probably will for ever 

 remain hidden in mystery and uncertainty. The supposition 

 that the Leptocardii and Cyclostomes, the lowest of the 

 .vertebrate series, must have preceded the other sub-classes, is 

 an idea which has been held by many Zoologists : and as the 

 horny teeth of the Cyclostomes are the only parts of their body 

 which under favourable circumstances might have ^ 



been preserved, Palaeontologists have ever been ^ 



searching for this evidence. p; ^o^ 



Indeed, in deposits belonging to the Lower Rigit dental 

 Silurian and Devonian, in Eussia, England, ? ^ ^J" • ^^" 



' ^ o ' xne amms. 



and North America, minute, slender, pointed 

 horny bodies, bent like a hook, with sharp opposite mar- 

 gins, have been found and described under the name of 

 Conodonts. More frequently they possess an elongated 

 basal portion, in which there is generally a larger tooth 

 with rows of similar but smaller denticles on one or both 

 sides of the larger tooth, according as this is central or at one 

 end of the base. In other examples there is no prominent 

 central tooth, but a series of more or less similar teeth is 

 implanted on a straight or curved base. Modifications of 

 these arrangements are very numerous, and many Palaeonto- 

 logists entertain still doubts whether the origin of these 







