836 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



unable to find any trace of it even in the oldest larva whose head 

 we have had (26 millims.), and .at a period when the gills on the 

 hinder arches have reached their full development. 



We imagined the gill in question to be the remnant of a gill 

 fully formed in extinct Ganoid types, and therefore expected to 

 find it better developed in the larva than in the adult. That the 

 contrary is the fact appears to us fairly certain, although we can- 

 not at present offer any explanation of it. 



SYSTEMATIC POSITION OF LEPIDOSTEUS. 



A. Agassiz concludes his memoir on the development of 

 Lepidosteus by pointing 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 Agassiz' opinion. 



Apart from the complete segmentation, the general develop- 

 ment 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 opportunity of 

 practically working at the subject, we may point to the following 

 developmental features 1 as indicative of Teleostean affinities : 



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

 the epiblast. 



(2) The division of the epiblast into a nervous and epidermic 

 stratum. 



(3) The mode of development of the gut (vide pp. 752 754). 



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

 as shewn on p. 822, the pronephros of Lepidosteus has primitive 

 characters not retained by Teleostei. 



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

 column (vide p. 779). 



In addition to these, so to speak, purely embryonic characters 



there are not a few important adult characters : 



(i) The continuity of the oviducts with the genital glands. 



1 The features enumerated above are not in all cases confined to Lepidosteus and 

 Teleostei, but are always eminently characteristic of the latter. 



