40 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



(1895), Cill (1895), Willey (1901), and Tattersall (1903"), and is defined 

 by all excepting Kirkaldy for the inckision of symmetrical species only. 



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

 nal form by Kii'kaldy (1895) and by Gill (l895), but expanded by 

 Tattersall (1903") to inchide all unsymmetrical species, whether they 

 possessed a urostyloid process or not. 



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

 Willey (1901) to generic value for the reception of all uusymmetrical 

 species, including those originally put under Asymmetron. 



These three genera may be briefly defined as follows : 



1. B ranch iosiojna. — 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 tlie 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 ; and a well developed urostyloid process is present. 



As already indicated, species representing all three genera were found 

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



Branchiostoma pelagicum Gunthee. 



Guntlier, 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 Timarafuri, Kolumadulu 

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

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

 specimen obtained by Mr. J. J. Lister, in the Indian Ocean midway l)etween 

 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 l)y Tattersall (1903'', p. 214) ; all 

 six were taken l)y tow-net in the Indian Ocean, one about halfway between 

 Perim 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. 



