1192 
SCANDINAVIAN FISHES. 
However patent these differences may appear, inter- 
mediate forms give rise to very great uncertainty. As 
examples we will here adduce the corresponding measure- 
ments in four Lamperns of the form usually called Petro- 
myzon Planer i, two males and two females 278 — 309 mm. 
long, taken together in Lake Ifo (Scania): 
Minimum. 
Maximum. 
Greatest height of the first dorsal fin in % of the total length of the head and branchial region 
9.i 
14.2 
„ „ „ „ „ „ ., ,, ,, „ „ distance between the first dorsal and the tin of the snout 
4.3 
6.5 
„ „ „ „ second „ ,, „ „ „ „ total length of the head and branchial region 
16.8 
21.7 
„ „ „ „ ,, „ „ „ ,, „ „ distance between the first dorsal and the tip of the snout 
7.8 
9.9 
and in four females of the form Petromyzon fluviatilis, 255 — 288 mm. long, taken together at Lulea: 
Minimum. 
Maximum. 
Greatest height of the first dorsal fin in % of the total length of the head and branchial region 
7.4 
9.9 
„ „ „ „ „ ,, ,, ,, ,, ,, ,, distance between the first dorsal and the tip of the snout 
2.8 
3.7 
„ „ ,, „ second „ „ ,, „ „ „ total length of the head and branchial region... 
14.4 
18.7 
„ „ „ „ ,, „ „ ,, „ ,, distance between the first dorsal and the tip of the snout 
5.6 
7.7 
As may be easily seen, these percentages partly 
overlap one another, partly approach one another in 
very close proximity. 
Equally variable is the character drawn from the 
relative position of the two dorsal fins. The length of 
the space between them varies in Petromyzon fluviatilis, 
according to Lonnberg’s measurements of 142 speci- 
mens 245 — 375 mm. long, between 1* *8 and 7'8 % of 
the length of the body; 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 Petromyzon fluviatilis approached 
nearer and nearer to the character of Petr. Planeri, 
but eventually receded all the more abruptly therefrom. 
Often, however, we find a dermal flap extending for- 
ward from the beginning of the 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" 1 , WaygeiA, and BeneckfA have totally re- 
jected the specific distinction between the two forms. 
The relation between them, as Lonnberg has remarked, 
is very like that between the large Trout ( grdlax ) 
and the Salmon ( blanklax ); and in the preceding pages 
we have made the same observation in many other 
parts of the history of the Scandinavian fishes. Fresh- 
water life has had greater influence on Petromyzon 
fluviatilis than on Petr, marinus; and two varieties have 
thus arisen, which in their typical forms are indeed 
easy of distinction, with the guidance of the above- 
mentioned characters, but are linked together by inter- 
mediate forms. “Among 7 specimens of Petromyzon 
Planeri from the same locality (Bieberbach, near Gies- 
sen) 3 had the two dorsal fins separated by an interval, 
while in 4 the fins were contiguous” (Schneider). 
The first dorsal fin begins in the Pride and usually 
too in Petromyzon Planeri somewhat in front of, in the 
typical Petromyzon fluviatilis somewhat behind, the 
middle of the body. The second dorsal begins in the 
former two in front off, in the last-mentioned form 
behind, the beginning of the posterior third of the body. 
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 
the latter fin, is more prominent in Petromyzon fluvia- 
a Beitr. Vergl. Anat., Entwickl. Wirbellh., p. 35, § 1. 
* Verb. K. K. Zool.-Bot. Ges. Wien, XXXIII (1884), p. 311. 
c Handb. Fiscliz., Fischer. (Max v. d. Borne), p. 193. 
d Exceptions not seldom occur in old specimens of Petromyzon Planeri. 
