306 J. W. KIRKALDY. 



14. Form of the anterior (rostral) and posterior (caudal) 

 expansions of the median fin. 



15. Number and myotomic position of the gonad pouches. 



The valuable diagnostic character afforded by the total 

 number of the myotomes, and of the number in the three 

 groups known as prseatrioporal, prseanal, and post-anal, was 

 first made use of by Sundevall and later adopted by Giinther 

 (No. 5), whose essay is up to the present time the most important 

 on the taxonomy of the Branchiostomidse. During the past 

 year, however, a very important memoir has appeared by 

 Andrews (No. 1) on a Branchiostomid from the neighbourhood 

 of the Bahamas, which presented so many novel features as 

 compared with the forms already known that it was necessary to 

 place it in a new genus, to which Andrews gave the unfortunate 

 name Asymraetron. Before the appearance of Andrews' 

 memoir Dr. Arthur Willey had, at Professor Lankester's 

 suggestion, undertaken the examination of Professor Haddon's 

 collection of Branchiostomidse from Torres Straits, and the 

 result was a short memoir (No. 14) in which some important 

 characters of B. cultellum, Peters (of which the entire col- 

 lection consisted), are pointed out; and the significant sugges- 

 tion is made that the median ventral fin of the Branchiosto- 

 midse is not truly a median structure, but is the continuation 

 of the right metapleuron. The two papers of Willey and of 

 Andrews made it desirable to re-examine as far as possible all 

 the species of Branchiostomidse, in order to ascertain whether 

 the peculiar features as to the unilateral character of the 

 gonads, continuity of metapleur and ventral median fin, absence 

 of ventral fin-rays in the one case and of ventral fin-ray 

 chambers in the other, described by these authors, obtain in 

 other forms which had not been looked at by their original 

 describers with these questions in mind. The nature of my 

 work is thus explained. Among the characters made use of, 

 I have added to those introduced into consideration successively 

 by Sundevall, Giinther, Willey, and Andrews, only one, viz. 

 the number of the cirrhi or tentacles lying on the inner face of 



