364 
SCANDINAVIAN FISHES. 
dorsal and anal tins begin to develop true rays; and 
finally the knob-like rudiments of the ventral tins ap- 
pear. Up to now the head has been perfectly sym- 
metrical. The eyes are perfect, with strong motory 
muscles, and have already begun to show that marked 
mobility which characterizes the Flatfishes in general, 
one eye lying on each side of the tumid middle brain. 
The covering bones of the brain-ca.se are as yet mere 
membranous rudiments. 
As the vertical fins now develop more and more, 
the caudal fin assumes its homocercal shape, the dorsal 
and anal fins are furnished with true rays and inter- 
spinal bones, and the body grows deeper. The young 
Flatfish now begins to lie often er on one side, and 
swims in a more and more oblique position. One of 
the eyes begins to turn inwards, first moving some- 
what forward towards the snout and inward towards 
the opposite side of the body (fig. 99, b, c and cl), 
then up the forehead and over to the opposite side 
(fig. 99, e), and finally, in most cases, further and 
further back on this side, which is now the eye side, 
while the other side of the body has become the blind 
side. Sometimes, e. g. in the genus Platophrys (Rhom- 
boid ichthys), the eye retires so far back that it appar- 
ently lies high up on the occiput. 
Meanwhile the covering' bones of the skull have 
begun to develop, but they are still cartilaginous and 
thin, and of far too soft texture to hinder the move- 
ments and the changes in the position of the eye, 
which, on the other hand, both fix the eventual shape 
of these bones and leave room for their abnormal, se- 
condary development. In the first place the front part 
of the frontal bones (fig. 100, / and /’) is compressed 
between the eyes (o and o'), while the posterior part 
of these bones (fig. 101, fr. d and fr. s, beneath in 
the figure) generally retain their breadth, but grow 
more and more distorted. The lateral ethmoid bones 
(the anterior frontal bones, fig. 100, a and a'; fig. 101, 
fr. d and fr. a. s), which are pierced by the olfactory 
nerves, are also removed and changed in development: 
the lateral ethmoid bones of the blind side (fr. a. s) 
and the true ethmoid bone (fig. 100, e; in fig. 101 not 
specially marked, but lying between fr. a. s and fr.a.cl ) 
form the usually raised, anterior and antero-exterior 
wall of the orbit, which is entirely closed on the outer 
(upper) side, this being due to the fact that, the frontal 
bones of the blind side (fr. s ) outside (above) the eye 
grow out anteriorly into a process which meets a cor- 
responding excrescence in a. backward direction of the 
lateral ethmoid bones of the blind side. Sometimes, 
however, these processes distinctly represent the sub- 
orbital bones®. They thus form an osseous bridge at 
the eventual dorsal edge of the head; and forward above 
this bridge, in most cases, grows the anterior part of 
the dorsal fin with its interspinal bones, which are 
supported thereon, just as the lateral muscles of the 
body (the occipital muscular part) extend forward to 
Fig. 100. Skull of a Cod, seen from above and reduced in size. 
After Steenstrup. v, vomer; e, ethmoid bone; a right, a' left lateral 
ethmoid bone; o, situation of the right, o' of the left eye; f, right, 
f left fronlal bone; p, posterior frontal bone on the right, p' on the 
left; c, upper occipital bone. 
this point. The occipital ridge of the skull is also 
continued in most cases on this osseous bridge, which 
thus, although secondary, and issuing from the original 
lateral parts of the skeleton, acquires the appearance 
of a median formation. It is true that the passage of 
the eye exercises some influence over the position and 
form of the other bones of the skull: the parasphenoid 
bone anteriorly, from the orbital region, becomes more 
or less twisted, or more developed on the one side of 
the body (the blind side), and in most cases the vomer 
also shares this rotation. But the bones of the true 
cranium (except the frontal bones) undergo only slight 
“ In Pleuronectes ajnoglossus tlie osseous bridge thus formed is furnished, on the blind side, with a coarse-meshed network of bony 
ridges, belonging to a muciferous duct; and in Solea ( Achirus ) lineata the bridge is double. The only other trace of a true suborbital ring 
that occurs, is the frequent presence of a preorbital bone united to the lateral ethmoid bone of the eye side. 
