HETEROPLEUKOiN HECTORI, NEW ZEALAND LANCELET. 275 



extent, though rather higher over its anterior quarter; while 

 shortly before the level of the atriopore it again gradually 

 rises to form the caudal fin. The fin rays cease at the level 

 of the anus. The rostral fin — as 1 have remarked — is only 

 indicated in the drawing with some hesitation ; but it appears 

 to be rhomboidal in outline^ rising suddenly from the dorsal. 

 The ventral fin, or pre-anal region of the median fin, is short 

 and low ; it contains about a dozen unpaired fin-ray boxes, 

 which are, however, without fin rays, as is the case, too, in H. 

 cultellum. At first these boxes are quite distinct, but 

 after twelve or fourteen complete ones the outlines become 

 less and less distinct, and soon disappear altogether. 



The caudal fin rises quite gradually from the dorsal, at 

 about the level of the atriopore, without any abrupt angle, 

 such as is seen in H. bassanum and H. cingalense; 

 but in this respect it resembles H. cultellum. Its lower 

 moiety is, howevei-, deeper than the upper, and rises rela- 

 tively far forwards — about the seventh myotome behind the 

 atriopore. 



It attains its greatest height at about the fourteenth 

 post-atriopoi-al myotome, that is some distance anterior to the 

 anus. Posteriorly the upper and lower margins slope 

 gradually, and equally and regularly backwards, and appear 

 to pass in the same curve to a point a short distance behind 

 the notochord. 



On examining the transparent specimen I noted a series 

 of sliort brownish chitinoid rods along the ventral base of 

 the caudal tin, extending outwards from the lower ends of 

 the muscles for a distance equal to about oue fourth the 

 depth of the fin (fig. 5). Each rod spreads out slightly near 

 its distal end, and becomes thinner and more transparent — 

 losing itself in the tissue of the fin. These rods I traced 

 backwards to the end of the body, whilst forwards there is 

 a great gap between tliem and the ventral fin-ray boxes, the 

 walls of which have quite a different aspect from the rods, 

 which do not appear to be optical sections of transverse 

 walls, but appear to be definite, solid, rod-like structures. 



