F. M. BALFOUR 283 



of the present Chaetopoda, but in a stock of segmented forms 

 descended from the same unsegmented types as the 

 Chcttopoda, but in which two lateral nerve-cords, like those 

 of Nemertines, coalesced dorsally instead of ventrally to form 

 a median nervous cord. This group of forms, if my sugges- 

 tion as to their existence is well founded, appears now to 

 have perished." x 



He held that while there was much to be said for the 

 interchange of dorsal and ventral surfaces postulated by 

 Dohrn and Semper, the difficulties involved in the supposi- 

 tion were too great; he preferred, therefore, to assume that 

 the present Vertebrate mouth was primitive, and not a 

 secondary formation. 



His views as to the phylogeny of the Chordata and the 

 genetic relation of the various classes to one another are 

 exhibited in the following schema, 2 names of hypothetical 

 groups being printed in capitals, names of degenerate groups 

 in italics : 



Mammalia. Sauropsida. 



PROTO-AMNIOTA. Amphibia. 



I _ I 



PROTO-PENTADACTYLOIDEI. 



Teleostei. 



Ganoidei. 

 I 



Dipnoi. 



PROTO-GANOIDEI. 



Holocephali. 

 E lasmobranchii. 



PROTO-GNATHOSTOMATA. 



Cyclostomata. 



PROTO-VERTEBRATA. 



r i 



Cephalochorda. PROTOCHORDATA. Urockorda. 



1 A Treatise on Comparative Embryology, vol. ii , p. 311, London, 

 1 88 1. 2 Loc. tit., vol. ii., p. 327. 



