1106 
SCANDINAVIAN FISHES. 
These lateral rows do not occupy the extreme side- 
margin of the tail, but the space between this and the 
median series, except where there are two lateral rows 
on each side, in which case the outer row projects from 
the said margin. They do not extend throughout the 
length of the tail, but only a short distance along it; they 
are not symmetrical, and contain an extremely variable 
number of aculei. A little behind the termination of 
these lateral rows and before the origin of the first 
dorsal fin, we usually find in old specimens 1 — 3 true 
lateral aculei (set at the extreme side-margin), in 
most cases the largest on the tail, seldom of equal 
number on both sides. These few lateral aculei at the 
said point and those occupying the space between the 
two fins are, though not constant, good characters, 
where they are present, of the Thornback. The extra- 
ordinary aculei are so variable, both in distribution 
and number, that no rule can be stated. They seem- 
ingly appear a short time before the period of puberty, 
for in all young specimens there is not even a sign of 
their presence. They are most numerous in middle- 
aged females, but in very old ones most of them have 
again disappeared". They are extremely seldom sym- 
metrical, sometimes preponderating in number on the 
dorsal side, sometimes on the ventral; sometimes scat- 
tered, sometimes collected in groups. Their basal scutes 
are often of considerable extent. Among the Scandi- 
navian species of the family the Thornback is the only 
one in which aculei appear on the ventral side. 
The coloration of the dorsal side is yellowish gray, 
with a number of large, round, whitish yellow spots of 
varying size and without symmetry. Between these 
appear smaller, black spots, like round dots, which 
gather round the former and compose black frames, 
sharply defined especially in young specimens. Some- 
times there also occur in old individuals large ocelli, cir- 
cular black spots with light margin, in one or two pairs 
on the dorsal side. The ventral side is white, with a 
dash of violet round the margins of the fins. On the 
tail the dark colour now and then, especially in young 
specimens, forms several (up to 6 — 8) broad transverse 
bands, most distinct on the under surface thereof. 
The sex is manifested externally, in part and with 
most clearness, by the large pterygopodia of the male 
and the cards of spines peculiar to him on the upper 
surface of the pectoral fins — two patches of erectile 
spines on the sides of the orbital region and another 
pair of larger patches within the outer angles of the 
fins — in part by several other differences already no- 
ticed. However distinct all these characteristics may 
be at an advanced age, during youth they are wanting 
or only subindicated, and at first the sexes are exter- 
nally indistinguishable. As the time of puberty ap- 
proaches, they all appear almost simultaneously, and 
afterwards the difference between the sexes grows more 
marked year by year. In a male 216 mm. long (pre- 
sumably a year old) taken in August, Malm estimated 
the length of the pterygopodia at only 3 mm. In a 
male about 42 cm. long Fries found the pterygopodia 
still quite small and short, their tips extending only a 
little more than half-way along the ventral fins; there 
were no signs of cards; the teeth were still of a gra- 
nular, lozenged and blunt form; the ventral side of 
the body was quite smooth, only a few spots (the tip 
of the snout and the sides of the belly) being rough 
to the touch ; there were no extraordinary aculei, and 
of the ordinary caudal aculei only the median row was 
present. In a specimen 52 cm. long the circumstances 
were essentially the same, only that the pterygopodia 
were somewhat longer in relation to the ventral fins, 
and the ventral side of the body was rougher, especially 
under the tail. But in an individual rather more than 
60 cm. long the pterygopodia are so developed that 
their tips extend more than half-way along the tail, 
and all the other characteristics of the male have ap- 
peared, though the cards are still small, containing 
few spines, and these set in 2 or 3 rows, whereas in 
larger and older specimens as many as 6 — 8 rows 
may be counted, each containing some twenty spines 
or more. 
The Thornback is the commonest of all the Swe- 
dish Rays. In the Sound it occurs southwards to the 
shallows off Saltholm (Winther). It has occasionally 
been met with in the south-west of the Baltic, on the 
east coast of Schleswig-Holstein (Schonevelde and 
Mobius and Heincke) and the north coast of Mecklen- 
burg (Boll*). It is most plentiful in the Cattegat 
and the North Sea. To the north, according to Storm, 
its range probably does not extend beyond the neigh- 
bourhood of Trondhjem. To the south it penetrates into 
° Fries met with very old males as well as females destitute of a single extraordinary aculens. 
6 Die Ostsee, Arch. Ver. Fr. Naturg. Meckl., Heft. I, p. 89 (sep.). 
