Benham. — On the Neiv Zealand Lancelet. 121 



metapleural ridge is directly continuous with the ventral fin, 

 the left ceasing at the atriopore. 



The genus Heteropleuron is the commonest in the southern 

 seas, and I was anxious to examine our lancelet in order to 

 ascertain to which genus it really belongs. Sir James Hector 

 was good enough to allow me to examine these interesting 

 and hitherto unique specimens, and I owe him my very best 

 thanks for so readily and generously sending the specimens to 

 Dunedin in accordance with my request ; and still further 

 do I thank him for permitting me to retain — as the type of the 

 new species — one of the two specimens, that which I had more 

 particularly and carefully examined and drawn. The other 

 specimen, now in the Colonial Museum, is thus a cotype. 

 The New Zealand lancelet is a very distinct species, and I 

 have named it Heteropleuron hectori. 



Heteropleuron hectori, n. sp. 



Length, just under 2 in. in the preserved condition, and 

 height | in. over greater part of body, tapering to each end. 

 Total number of muscle-segments, as indicated by the angu- 

 lated lines at the sides of the body, 84 (or possibly 85, the 

 first and last being very small), of which 53 lie in front of the 

 •atriopore, 19 (or perhaps 20) between atriopore and anus, and 

 12 behind the anus. The " myotome formula," therefore, is 

 53 + 19 (or 20) + 12 = 84 (85). 



The median fin is low over the greater part of its dorsal 

 extent ; it enlarges to form a small rostral fin (in front of the 

 mouth), which, however, is injured in both specimens, so that 

 its true outline is somewhat uncertain. 



The caudal fin, or posterior expansion of the median, is 

 relatively large ; it commences dorsally at a point about mid- 

 way between vertical lines through the atriopore and anus — 

 i.e., about the 10th postatrioporal myotome ; and ventrally it 

 commences at about the 7th postatrioporal myotome. It soon 

 attains its greatest height at about the 14th myotome — i.e, in 

 front of the anus — and thence tapers regularly and gradually 

 to a point a short distance beyond the tip of the notochord. 

 Its margin is rounded, and not angulated as in B. lanceolatum. 

 Our species, in the shape and proportions of the caudal fin, is 

 very different from either of the species of Heteropleuron oc- 

 curring in the Australian seas. Thus, in H. bassanum, from 

 Bass Strait, the caudal fin commences behind the anus ; while 

 in H. cultellum, from Torres Strait, the caudal, although 

 commencing before the anus, attains its greatest height at 

 the level of this aperture ; but in myotome formula and in 

 :size these differ considerably from our species. 



The ventral fin — i.e., that part of the median fin lying 

 between atriopore and the origin of the caudal — is scarcely to 



