CLASSIFICATION OF THE CEPHALOCHOUDA. 277 



as themselves generically distinct, for the only difference 

 between the two, namely the presence of oral cirri in 

 Branchio stoma and their absence' in Amphioxides, 1 is not 

 enough to justify a generic separation^ and, therefore, I 

 agree with GrtLnther in retaining this species in the genus 

 Branchiostoma. 



Similarly the three genera of Gill, Paramphioxus, 

 Epigonichtliys and Asymmetron, while distinct as a group 

 from the other two genera, can hardly be considered as 

 generically distinct from one another. The differences 

 between them are solely the character and shape of the 

 fins. 



If we consider Kirkaldy's classification it is at once 

 obvious that the sub-genus Heteropleuron is more nearly 

 related to the genus Asymmetron than to the genus 

 Branchiostoma in having uniserial gonads and asym- 

 metrical nietapleura., the two chief characters of Asym- 

 metron. In defining the characters of his new genus, 

 Andrews regarded the above two characters only as being 

 generic ; the other points in which the new form differs 

 from other then known forms he considered as only of 

 specific value. Kirkaldy on the other hand regards these 

 latter characters as of generic value and considers 

 Andrew's generic characters of only sub-generic rank. 

 Willey returns to Andrew's view and in his classification 

 places both Asymmetron and Heteropleuron under one 

 genus defined by the above two characters. The 

 differences between the two he considers to be of sub- 

 generic value. His classification, however, takes no 

 cognisance of Kirkaldy's new species Heteropleuron 

 cingalense, and he neither proposes a new sub-generic 

 name nor indicates where he would place the species. 



1 See later for reasons suggesting that the absence of oral cirri in 

 B. pelagicum is the result of pelagic life. 



