59 



which thirty-seven are anterior to the atriopore , sixteen between the 

 atriopore and the anus and eleven posterior to the anus. These num- 

 bers were found to be the same on the right and on the left in each 

 case. The length of the specimens was 35-5 mm, 45 mm, 29'5 mm, 

 45 mm, 31 mm and 46 mm in the six specimens: that is an average of 

 39 mm nearly. As far as can be judged from these specimens the for- 

 mula for the species would thus be: 37 • 16 • 11 — 64, 39 mm. 



This we see does not agree with that of any of the known forms, 

 but we need not conclude that it is new on that account since the for- 

 mulas are but approximately accurate. They are averages, often from 

 only a very few specimens and the actual numbers may vary consider- 

 ably in a given species. Thus in B. cultellum Willey^ counted 

 32 . 12 • 8 — 52 and 33 • 11 • 11 — 55 and again in some cases a total 

 of 54. Then again the errors in counting are large owing to the 

 difficulty'of locating the atriopore and anus accurately and to the diffi- 

 culty in seeing the boundaries of the last segments. 



The entire number of segments in the Japanese specimens is al- 

 most the same as in the formula for B. Belcheri while the number in 

 the anterior region is thirty seven in each case : there is thus a closer 

 agreement here than with any other species. We know, unfortunately, 

 very little of this B. Belcheri. Günther counted 37 • 14 • 13 — 64 in 

 specimens from Borneo and 37 • 14 • 14 — 65 in others from Prince of 

 Wales Islands : the former thus agree more closely with the Japanese 

 form as far as these numbers are indications. The other characteristics 

 of B. Belcheri as given by Gray and by Günther do not add very 

 much to our knowledge of this species. The original description by 

 Gray, in 1847, gives but a faint idea of the creature. He says it is 

 very much like Amphioxus lanceolatus, but thinks it more convex on 

 the sides, with the dorsal fin higher and with more numerous septa. 

 In 1884 Günther described it as »very similar to but more elongated 

 than B. Icmceolatum and the fins instead of being dilated behind gradu- 

 ally decrease in width towards the extremity of the tail«. 



In the Japanese specimens, however, the fins are not like this 

 but rather markedly dilated. The dorsal fin is much more suddenly 

 dilated at a point vertically above the anus than in B. lanceolaium. 

 The ventral fin is dilated much as in some specimens oî B. lanceolatum 

 from Sicily. This dilatation of the fins, however, can scarcely be relied 

 upon as of enough value to exclude the Japanese specimens from the 

 species B. Belcheri, for in another form, the B. californiense from Cali- 

 fornia, the tail fins present very difi'erent amounts of expansion in dif- 

 ferent specimens preserved by different methods. It may then be that 



2 Quart. Journ. Micr. Sc. January 1894. 



