The Ganoids 5 



Admitting all this, it is nevertheless natural and convenient 

 to retain the Ganoidei (or Chrondrostei if the older name be 

 discarded on account of the many meanings attached to it) 

 as a group equivalent to that of Teleostei within the class or 

 subclass of Actinopteri. It comprises the transitional forms 

 between the Crossopterygii and the bony fishes, and its members 

 are especially characteristic of the Mesozoic age, ranging from 

 the Devonian to the present era. 



Of the extensive discussion relating to this important ques- 

 tion we may quote two arguments for the retention of the sub- 

 class of Ganoids, the first by Francis M. Balfour and William 

 Kitchen Parker, the second from the pen of Theodore Gill. 



Balfour and Parker (" Structure and Development of Lepi- 

 dosteus," pp. 430-433) thus discuss the 



Systematic Position of Lepidosteus. "Alexander Agassiz con- 

 cludes his memoir on the development of Lepidosteus by point- 

 ing out that in spite of certain affinities in other directions 

 this form is 'not so far removed from the bony fishes as has 

 been supposed.' Our own observations go far to confirm Agas- 

 siz's opinion. 



"Apart from the complete segmentation, the general de- 

 velopment of Lepidosteus is strikingly Teleostean. In addition 

 to the general Teleostean features of the embryo and larva, 

 which can only be appreciated by those who have had an oppor- 

 tunity of practically working at the subject, we may point to 

 the following developmental features * as indicative of Teleos- 

 tean affinities: 



" (i) The formation of the nervous system as a solid keel 

 of the epiblast. 



" (2) The division of the epiblast into a nervous and epi- 

 dermic stratum. 



" (3) The mode of development of the gut. 



"(4) The mode of development of the pronephros; though 

 the pronephros of Lepidosteus has primitive characters not 

 retained by Teleostei. 



" (5) The early stages in the development of the vertebral 

 column. 



* The features enumerated above are not in all cases confined to Lepidos- 

 teus and Teleostei, but are always eminently characteristic of the latter. 



