34 



RECORDS OF THE AUSTRALIAN MUSEUM. 



The rostral fin is slightly expanded, not separated off from the 

 dorsal. The dorsal tin is moderately developed, not more than 

 one-seventh of the whole vertical diameter. The caudal fin is 

 distinctly lancet-shaped. The notoehord is produced a little 

 behind the last myotome. The ventral finis without rays. The 

 dorsal is without rays behind about the fortieth myotome. There 

 are twenty-five to thirty oral cirri, united at their bases by a web. 

 The velar tentacles are ten to twelve. 



Fig. 1. — Heteropleuron hedleyi, Haswell. 

 N.CH., anterior end of notoehortl ; R.F., rostral fin ; V.T., velar tentacles. 



When the Murray Island specimens of Heteropleuron are 

 compared with Kirkaldy's description and figure of H. cultellum, 

 and with my specimens of the latter species obtained in Torres 

 Strait during the cruise of H.M.S. " Alert," a very marked 

 difference is at once recognisable. Apart from the myotome 

 formula and the number of gonads, the high dorsal fin quite 

 definitely separates H. cultellum from all other species of 

 Amphioxus. From II. bassanum the Murray Island form is also 

 quite clearly separated by different and constant, though less 

 striking, differences. H. bassanum has about seventy-five myo- 

 tomes or more and twenty-six to thirty-one gonads ; it has paired 

 fin-rays in the ventral fin, and it has the rostral fin separated off 

 from the dorsal by a marked "dip." //. cinyalenxe, the only 

 other described species of Heteropleuron, has sixty-one to sixty-four 

 myotomes, with the formula 39 or 31 +17 or 16 + 6 or 8. The 

 ventral fin has paired fin-rays and there are twenty-five gonads. 



Altogether six species of Amphioxus are now known to be 



in Australian seas. These are: — Heteropleuron bassanum, Gim- 



