1192 



SCANDINAVIAN FISHES. 



However patent tliese differences may apjjear, inter- \ inents in tour Lamperns of the form usnally called Fetro- 

 luediate forms give rise to very great uncertainty. As mijzon Planeri, two males and two females 278 — 309 mm. 

 examples we will here adduce the corresponding measure- long, taken together in Lake Ifo (Scania): 



and in four females of the form Fetroini/zon Jiuciatills, 255 — 2S8 mm. long, taken together at Lulea: 



As may he easily seen, these percentages partly 

 overlap one another, partly approacli one another in 

 very close proximity. 



Equally variable is the character drawn from the 

 relative position of the two dorsal tins. The length of 

 the space, between them varies in Petroniyzonfluviatilis, 

 according to Lonnberg's measurements of 1-12 speci- 

 mens 245 — 375 mm. long, between 1'8 and 7'8 % of 

 the length of tlie l)ody; and on examining the results 

 of these measurements we find that in the 12 smallest 

 (245 — 255 mm. long) the average percentage was 4'5, 

 in the next 65 specimens (260 — 295 mm. long) 4'3, 

 in the next 52 (300 — 335 mm. long) 4"1, and in the 

 13 largest specimens (340 — 375 mm. long) 4"7. During 

 the period of growth examined by Lonnberg it thus 

 appeared that at first Petromi/zon fluvlafiUs approached 

 nearer and nearer to the character of Petr. Planeri, 

 but eventually receded all the more abruptly tlierefrom. 

 Often, however, we find a dermal flap extending for- 

 ward from the beginning of tht- second dorsal fin in 

 the median dorsal line towards the first dorsal, so that 

 the limit of the former fin is difficult to fix. And 

 Schneider", Waygel'', and Benecke" have totally re- 

 jected the specific distinction between the two forms. 



The relation between them, as Lonnberg has remarked, 

 is very like thtit between the large Trout (grdlax) 

 and the Salmon (blanklair); and in the preceding pages 

 we have made the same observation in many otJier 

 parts of the history of the Scandinavian fishes. Fresh- 

 water life has had greater influence on Petromyzon 

 fluviatiUs tlian on Petr. marbius: and two varieties have 

 thus arisen, which in tlieir typical forms are indeed 

 easy of distinction, with the guidance of tlie above- 

 mentioned characters, but are linked together by inter- 

 mediate forms. "Among 7 specimens of Petromyzon 

 Planeri from tiie same locality (Bieberbach, near Gies- 

 sen) 3 had the two dorsal fins separated by an interval, 

 while in 4 the fins were contiguous" (Schneider). 



Tlic first dorsal fin begins in the Pride and usually 

 too in Petromyzon Planeri somewhat in front of, in the 

 typical Petromyzon fluviatiUs somewiiat l)eliind, the 

 middle of the body. The second dorsal begins in the 

 former two in front of, in the last-mentioned form 

 beliind, tlie beginning of the posterior third of tlie Ijody. 

 The caudal fin is of the same symmetrical form as in 

 the preceding species. The boundary between it and 

 the second dorsal, marked by the more distinct rays of 

 tlie latter fin, is more prominent in Petromyzon fliivia- 



'• Btitr. VergL Aunt., Entwickl. Wirhellh., p. 35, § 1. 



» Verb. K. K. Zool.-Bot. Ges. Wien, XXXIII (1884), p. 311. 



' Handb. Fischz., Fischer. (Max'v. d. Bohne), p. 193. 



'' Exceptions not seldom occur in old specimens of Petromyzon Planeri. 



