40 BULLETIN : MUSEUM OF COMPAEATIYE ZOOLOGY. 



(1895), Gill (1895), Willey (1901), and Tattersall (1903'), aud is defiued 

 by all excepting Kirkaldj for the inclusion of symmetrical species only. 



The second is Andrew's Asymmetron, which was accepted in its origi- 

 nal form by Kirkaldy (1895) and by Gill (1895), but expanded by 

 Tattersall (1903*) to include all unsymmetrical species, whether they 

 possessed a urostyloid process or not. 



The third is Kirkaldy's subgenus Heteropleuron, which was raised by 

 AVilley (1901) to generic value for the reception of all unsymmetrical 

 species, including those originally put under Asymmetron. 



These three genera may be briefly defined as follows : 



1. Branchiostoma. — Symmetrical cephalochordates in which the right 

 and left metapleural folds terminate just behind the anus and the gonads 

 form two lateral series. 



2. Heteropleuron. — Unsymmetrical cephalochordates in which the 

 left metapleural fold terminates just behind the anus, and the right one 

 is continuous with the median ventral fin ; the gonads form a single 

 series on the right side ; and a urostyloid process is not present. 



3. Asymmetron. — Unsymmetrical cephalochordates in which the left 

 metapleural fold terminates just behind the anus and the right one is 

 continuous with the median ventral fin ; the gonads form a single series 

 on the right side ; aud a well developed ui'ostyloid process is present. 



As already indicated, species representing all three genera were found 

 by Mr. Agassiz in the Maldives, and their descriptions follow. 



Branchiostoma pelagicura Guxtheb. 



Giintlier, 1889, p. 43- 



Plate 1, Figs. 1, 3. 



A single specimen of this somewhat rare species was taken in the net between 

 one hundred and fifty fathoms and the surface at Tiniarafuri, Kolumadulu 

 Atoll. This species was originally described from a single specimen taken by 

 the "Challenger" a few degrees north of Honolulu in the Pacific. A second 

 specimen obtained by Mr. J. J. Lister, in the Indian Ocean midway between 

 Madagascar and Australia, was reported on by Cooper (1903, p. 352) in his 

 account of the Maldive and Laccadive cephalochordates. Finally six specimens 

 formed the basis of an account of this species by Tattersall (1903'', p. 214) ; all 

 six were taken by tow-net in the Indian Ocean, one about halfway between 

 Perini and the Maldives, and the other five between the Maldives and the Gulf 

 of Manaar. Notwithstanding these several catches, all three lots of material 

 were reported as poorly preserved, and consequently the best description of 

 this species that could be compiled is still defective in several important 

 particulars. 



