— 4 — 



occurring is entirely independent of the geographical distribution of the samples 

 within the great area extending from Iceland to the eastern waters of the Mediter- 

 ranean. 



A comparative investigation of the ano-dorsal distance for a large number of 



samples shows a slight increase in the value of - ■ 100 from north to south. 



The immediate cause or causes of this peculiarity I was not able to ascertain; I 

 did, however, arrive at the conclusion that it seems impossible to split up Anguilla 

 vulgaris into two or more races on the basis of this variation, since the extreme 

 values are gradually connected by all intermediate values. Of other variable cha- 

 racters, the length of the head I - • 100) and the length of the elver stage were briefly 



referred to. 



In the first report, the investigations there described were expressly designated 

 as a first contribution towards the solution of the problem in question, and it was 

 predicted that several other characters would have to be included in the work 

 before this could be expected to furnish any definite result. 



The present work is thus a continuation and amplification of the investigations 

 commenced in the first as to the classification of the fresh-water eel. The same 

 three species are here dealt with: Anguilla vulgaris, Anguilla rostrala and Anguilla 

 japonica. In the case of the American species, the material has been essentially 

 increased, a large sample of elvers collected at W. Gloucester, Mass. in May 1913 

 by Mr. W. W. Welsh having been received from Dr. Hugh M. SMrrH of Washington 

 D. C, U. S. A. Thanks to this sample, I have been able to deal with the American 

 eel far more thoroughly than hitherto. Further assistance in this respect has also 

 been furnished by a small sample of elvers and young eels collected at the begin- 

 ning of 1914 by the staff of the Danish Commission for the Investigation of the 

 Sea at St. Croix, in the Danish West Indies. 



In the rest, the material employed has been essentially the same as that on 

 which the frequently mentioned Report was based. This last, however, .was chiefly 



concerned with two characters, viz. : the number of vertebrae, and — - — • 100 ^ whereas 



in the present treatise, a number of other characters have been included. By this 

 means, the results have been rendered considerably more reliable. 



The characters here particularly dealt with are the so-called permanent charac- 

 ters, (for definition of the term, vide Report I, p. 4 — 5). These are as follows: 



1) Number of vertebrae (supplementary to the matter contained in Report 

 I, 1913). 



2) Number of anal rays. 



3) Number of branchiostegal rays. 



4) Number of pectoral rays. 



5) Number of caudal rays. 



'■ Other characters were, however, occasionally referred to, as for instance the length of the 

 leptocephalus stage (p. 25) and the elver stage (p. 23), the length of the head (p. 21) the number of 

 pectoral rays (p. 6) and anal rays (p. 6). 



