DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 285 
canal of the eyeless side having completely disappeared in all the species I have ex- 
amined. In Platessa limanda the.turbinal ossicle of the eyeless side is about as small 
as in the Plaice; in P. flesus it is a little larger. 
But the most remarkable condition of the mucus-canals in the genus Platessa, and 
indeed in the whole group of flatfishes as far as I know, is that seen in Platessa pola*. 
In this fish we have on the ocular side (Plate XXXI. fig. 5) nothing very peculiar to 
notice: the arrangement seems to be just as in the Plaice, excepting that there is no 
cross commissure. The suborbital and supratemporal bones are very small and delicate, 
and generally have not closed over so as to form complete tubules. But on the eyeless 
_ side the mucus-canals are dilated into large, circular, flattened ampullæ, the outline of 
which I have given in Plate XXXI. fig. 6. These ampullæ are twenty-six in number ; 
six of these are situated on the main trunk, two on the detached nasal portion, four on 
the supratemporal branch, eight on the operculo-mandibular, and six on the suborbital. 
The supporting bones of this system are also peculiarly modified: the suprascapular, 
mastoid, frontal, and preopercular bones, also the lower jaw, are furnished with excava- 
tions for the support of the ampulle belonging to the main canal and the opercular 
mandibular branch. The supratemporal bones (2) five in number, are very delicate 
laminæ of bone pierced with many minute holes (when macerated), their lateral edges 
folded in, and connected by a bridge across the middle, where the whole bone is con- 
stricted. The hollow of each bone is therefore hourglass-shaped, and takes part in the 
Support of two ampullæ, which communicate by the narrow part passing Fig. 6. 
beneath the bridge aforesaid, as in the adjoining diagram. The os termis “47> f 
nale, or turbinal, has the same structure, as likewise have the five sub- 
orbitals, except the anterior one, which is somewhat trigonal, and takes N 
part in the formation of three ampullæ. I have said that there is no cross commissure ; 
the branch (e), on the ocular side, analogous to the commissural branch in the Plaice, 
ends almost immediately in a blind point on the eyeless side; the anterior ampulla on 
the main canal, indicated in dotted outline in fig. 6, is situated more deeply than the 
rest, being partly overlapped by the ampulla behind, and by the anterior two supra- 
temporal ones in front. This arrangement may be regarded as an indication of a com- 
Missure, but none is really effected. 
"d 
IV. Vertebral Column. 
The vertebra] column in the Pleuronectidz is usually supposed to be quite symmetrical. 
some it is not so, however, as I will presently point out. 
The vertebral column of the Plaice displays the following peculiarities :— 
- The mesial vertical plane of the vertebræ is curved, the convexity being toward the 
ed side, the concavity towards the eyeless. This is most strikingly seen in the anterior 
Yertebræ, such as the first, which I have represented (Plate XXXII. fig. 6) as seen from ~ 
* ; E : 
b Eds ampullated condition of the mucus-canals on the eyeless side of the head in P. pola has been war noticed 
wes ee Donnel in the "Trans, Royal Irish Academy,’ vol. xxiv, Science, 1862. On the morphological arrange- 
of these ampulle, he has, however, made no observations. 
