1 U) PALAEONTOLOGY. 





which the vertebral column terminates at the middle of the 

 base of the caudal fin. There are also intermediate forms 

 and structures of this fin, some of them exemplifying arrested 

 stages in the development of the homocercal tail. 



Fig. 58. 

 Heterocercal tail (Lepidosteus osseous). 



The fossil remains of the singular fishes of the extinct 

 order Placoganoidei were first discovered about 1813, in 

 formations of the " old red " or Devonian age in Russia, 

 and are preserved in museums at St. Petersburg and Dor- 

 pat. The relation of these specimens to the class of fishes 

 was first announced by Professor Asmuss,* and shortly after, 

 the generic names Asterolepis and Bothriolepis were invented 

 by Professor Eichwald,f to express certain modifications of 

 the external surface of portions of the ganoid plates, subse- 

 quently recognized as constituting the buckler of the fore-part 

 of the extinct fishes. In September 1840 Hugh Miller sub- 

 mitted to the geological section of the British Association at 

 Glasgow the first discovered specimens which afforded a recog- 

 nizable idea of the form of one of these " old red " fishes, and 



* Bulletin Scient. par l'Acad. Imp. des Sciences de St. Petersburg, 1840, t. 

 vi., p. 220. 



f Ibid, t. vii., p. 78, communicated March 13, 1840. Dr. Fleming had 

 recognized certain fossil scales as those of fishes in the "Old Red" of Fife- 

 shire, in 1827. 



