286 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 
the front. The spinous process is seen to be strongly bent over towards the eyeless side; 
a similar curve is seen affecting the middle line of the posterior aspect of the cranium 
(fig. 5). 
This condition gradually diminishes posteriorly; but throughout the entire series of 
caudal vertebræ the superior and inferior spinous processes are set at a slight angle to 
each other, consequently the entire skeleton of a Plaice is convex on the ocular surface, 
concave on the eyeless. 
2. The transverse processes of most of the abdominal vertebræ are unsymmetrical, 
slightly in their place of origin on each side from the bodies of their vertebræ, consider- 
ably so as regards the direetion in which they proceed. Seen from below, these processes 
on the eyeless side arise a little further forwards on the bodies of their vertebræ, and 
project nearly directly outwards, sometimes even a little anteriorly; while those of the 
ocular side are direeted considerably backwards. The four anterior vertebræ and the 
thirteenth (last abdominal) are pretty exempt from this condition; but it affeets the 
intermediate ones pretty strongly. Plate XXXII. fig. 8, shows the under aspect of the 
series of abdominal vertebræ in the Plaice, and how a line joining the tips of the trans- 
verse processes of one of the middle vertebræ of that series passes very obliquely across 
the long axis of the column. Again, when seen from before backwards, the transverse 
processes of the abdominal vertebre project more vertically downwards on the eyeless 
than on the eyed side—a circumstance in accordance with the more flattened shape of the 
fish on the former side. The transverse processes of the caudal region, in accordance 
with the well-known greater development of the lateral muscle on the ocular side, are 
also more prominent on the same side. 
The vertebrz of the Halibut present the same sort of asymmetries which we have 
observed in the Plaice: some differences are to be observed in the Turbot and Brill. 
Here asymmetry is chiefly to be seen in the transverse processes, which agree with those 
of the vertebræ of the Halibut and Place in this, that those of the abdominal region are 
more directed vertically downwards on the eyeless side, and those of the caudal region 
are on the same side less prominent. But, when looked at from above or below, the 
transverse processes in the abdominal region are seen to project more posteriorly on the 
eyeless side—a condition exactly opposite to that found in the Plaice. 
V. The Dorsal Fin.— Conclusion. 
The dorsal fin, it is well known, extends in all the Pleuronectidæ all along the back, 
and advances forwards on the top of the head. Its advance on the head depends on wo 
circumstances, 
l. A more and more oblique direction forwards of the anterior interspinous bones, 
till the first one, becoming horizontal or nearly so, carries the anterior rays of the fin 
to opposite the middle of the upper eye (Hippoglossus Platessa) or to beyond both 
(Rhombus Solea, &e.). 
bodily advance forwards of these interspinous bones themselves on the top of the 
um, 
In the Sole only about five interspinous bones arise on the top of the eranium, and their 
