CLASSIFICATION OF THE CEPHALOCHORDA. 273 



1889, B. pelagicum, for a surface form taken in mid-ocean 

 during the " Challenger " expedition. 



This brings us to 1893 when Andrews 1 described a new 

 genus from the Bahamas which he called Asymmetron 

 lucayanum. His minute and masterly investigation of 

 this form disclosed two new and important characters, 

 viz., a single row of gonads and the asymmetrical 

 character of the metapleural folds, the left stopping at 

 the anus, while the right was continuous behind with the 

 ventral median fin. 



These characters are at the same time very constant 

 and very easily made out. Moreover, Andrews founded 

 his new genus on them alone. The other points in which 

 it differed from known forms, he regarded as merely 

 specific differences. His new genus was, therefore, 

 soundly based on important and constant characters. 

 Willey, 2 in the same year, during an examination of a 

 collection of B. cultellum from Torres Straits, Australia, 

 noticed in this species the uniserial gonads, but did 

 not agree with Andrews in regarding them as 

 forming a generic distinction, and so retained the 

 species in the old genus Branchiostoma. He apparently 

 did not notice the asymmetrical metapleural folds, or, in 

 all probability, he would have regarded the form as 

 generieally distinct. In 1894 Miss Kirkaldy revised the 

 whole of the group, and her preliminary report was read 

 to the British Association in that year, 3 the final paper 

 appearing next year 1895. 4 Her classification was as 

 follows : — 



1 Stud. Biol. Lab. Johns Hopk. Univ. Vol. v., pt. 4. 1893. 



2 Quart. J. Micr. Sci. Vol. xxxv. 



» Brit. Assoc. Report. Oxford. 1894. 



4 Quart. J. Micr. Sci. Vol. xxxvii. 



