44 bulletin: museum of COMPAKATIVE ZOOLOCxY. 



simple outline from the more lancelike form of that of H. bassanum. Perhaps 

 the best differential character lies in _the ventral fin. In H. bassanum and 

 H. maldivense the ventral fin has fin-ray chambers and fin-rays ; in H. agassizii 

 it has no chambers and no fin-rays. A summarized statement of the contrasts 

 between H. agassizii and other allied species is given in Table 2, page 45. 



Heteropleuron parvum, sp. nov. 



Plate 3, Fig. 6. 



Two specimens of this species were dredged in sixteen fathoms of water at 

 Hanimadu, Tiladummati Atoll. They measured 11.5 mm. and 12.5 mm. in 

 length respectively, and their depth is about one-tenth their length. 



The dorsal fin is well developed, and is slightly higher posteriorly and espe- 

 cially anteriorly than in the middle. It has well marked fin-ray chambers ex- 

 tending from the anterior end of the nerve tube to the tail, and numbering 

 about four to a myotome. In the middle region the chambers reach the free 

 edge of the fin. Single fin-rays are present. Anteriorly the dorsal passes into 

 the rostral fin, posteriorly into the inconspicuous caudal. The caudal fin, 

 which is in no way marked off from the dorsal, is also continuous with the 

 ventral, which contains a series of low fin-ray chambers and short single 

 fin-rays. These are inconspicuous, and have not been shown in the figure 

 (Plate 2, Fig. 6). 



The myotome formula in both specimens is 40 4- 18 -1- 10 = 68. 



The chorda is stout and almost reaches the anterior and posterior limits of 

 the body. 



The nerve tube contains a conspicuous anterior eye-spot, and from the third 

 myotome to the last a series of smaller spots. 



The gonads, which were completely present in only one specimen, formed a 

 single series on the right side, and were sixteen in number. They extended 

 from the fourteenth to the thirtieth myotome. 



The structure of the ventral fin allies this species with the first four men- 

 tioned in Table 2. It dift'ers from all these in its small size and narrow form, 

 for it is even longer in proportion to its depth than H. bassanum, the narrowest 

 of the four. From H. bassanum it differs markedly in its myotome formula, 

 the small number of its gonads, and the simpler form of its tail. 



A comparison of H. parvum and H. agassizii with other allied species is 

 given in Table 2. 



