1102 
SCANDINAVIAN FISHES. 
induced by age. Judging by those species whose de- 
velopment Fries had an opportunity of tracing, he 
remarked, as a rule probably of general application, 
that no species of Ray belonging to the type with an 
odd number of rows of caudal aculei, when young or 
newly hatched, has more than one such row, which 
then invariably occupies the median line of the tail. 
The lateral rows do not appear until the fish has 
reached a more advanced age. Most of the Scandina- 
vian species fall under this type, but. develop in a di- 
rection involving something peculiar to and characteristic 
of each. In some the lateral rows always project from 
the side-margin of the tail, in others between this mar- 
gin and the median line. In certain forms the spines 
of the lateral rows never attain the same size and 
strength as those of the median row, while in others 
the relations are reversed. In some species the spines 
of the median rows are persistent even at a very ad- 
vanced age, in others they are normally lost or shed. 
The form of the jaw-teeth, remarked Fries, has 
been rejected as a specific distinction by Cuvier and 
other writers, and on the whole with good reason, for 
greater differences of dentition may often be observed 
between the young and the old, between the male and 
the female, than can be determined between two nearly 
related species. But it is equally certain, he wrote, 
that most of the species, not to say all the Scandinavian 
ones, have a fixed form of dentition proper to them, 
whereby some of them can safely be distinguished, and 
some — e. g. the Thornback, Shagreen Skate, and Starry 
Ray — are so well marked in this respect that they can 
be confounded with no other species. To gain a right 
conception of the dentition of each species, the deve- 
lopment of the teeth must be carefully followed, and 
a good clue is afforded to the investigator by a com- 
parison of the teeth nearest the corners of the mouth 
with those in the middle of the jaw. The former are 
arrested, as it were, at a lower grade of development, 
and most nearly resemble the teeth of the young; the 
latter exhibit the highest stage in the dental develop- 
ment of the species. The several intermediate rows 
between the said points show the transitions from the 
lowest to the highest development of the teeth. If at- 
tention be paid to this, and the gradual detrition to 
which the outermost (foremost) transverse rows in the 
mouth are subjected be also taken into consideration, 
the greater number of the said anomalies in the denti- 
tion of a species disappear. As in the preceding fa- 
mily, the males are characterized by more pointed jaw- 
teeth than those of the females; but only in one of the 
species indigenous to Scandinavia (the Thornback) is 
the dentition of each sex thoroughly distinct; in the 
others the difference is sometimes so slight that it has 
even been overlooked. 
The form of the snout affords characters of no 
little importance and constancy, especially if the alte- 
rations of growth be kept in mind ; and the variations 
in the form of the snout are attended by differences in 
the general configuration of the body, which form is 
really determined by that of the snout and the pectoral 
fins. To find a safe expression for these variations in 
the form of the snout, Fries compared the length of 
two lines, one drawn right across the head through the 
centre of the pupils, the other at right angles to this 
and extending to the tip of the snout. The species in 
which the latter line is less than half as long as the 
former, he called blunt-snouted; those in which the 
length of the snout is more than half or at least half 
the breadth of the head at the said point, he ranged 
among the sharp-snouted Rays (the Skates). Another 
expression of this, which besides affords an often re- 
quisite character from the under surface of the head, 
may be obtained by comparing the least width between 
the inner margins of the nostrils (the base of the nasal 
valvule) with the distance between the nostrils and the 
tip of the snout, whereby we find that in the blunt- 
snouted species the length of the base of the nasal 
valvule is more, in the sharp-snouted species less, than 
70 % of the distance between the nostrils and the tip 
of the snout. The form of the snout also supplies an- 
other character, which was indeed remarked by Fries 
in his descriptions, but has won greater recognition in 
more recent times". In the Rays with a very pointed 
snout a line drawn from the tip of the snout to the 
anterior margin of the outer tip of the pectoral fin falls 
entirely outside the disk, whereas in the blunt-snouted 
Rays it at least partly cuts the same. 
From the position of the dorsal fins with relation 
to each other Nilsson 6 deduced a specific character the 
value of which was also appreciated by Fries. Some 
species, such as the Starry Ray, have the two dorsal 
fins sel quite close together on the tail and without 
a See E. Mokeau and D6derlein. 
b Prodr. Ichthyoi. Scand., p. 119. 
