DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 275 
isstill less developed, and, the primordial cartilage in this region being less extensive than 
in the Turbot, a space is left in the anterior wall of the orbit, which is filled up by a de- 
velopment from the corresponding side of the nasal bone. (Compare the boundaries of 
the orbit in Plate XXIX. fig. 1 and in fig. 8.) 
In the cranium of Platessa pola.the interocular process of the frontal of the eye- 
less side presents a form intermediate between its condition in the Halibut and that 
in the Plaice, next to be described. It is continued between the eyes as a very slender 
spiculum, much more delicate than the corresponding part in the Halibut, and very apt 
to be broken off in disartieulating the skull. The external angular process of the same 
bone is very largely developed, and, with the corresponding part of the prefrontal of the 
eyeless side, forms the pseudomesial bar of the eranium into an expanded and flattened 
vertical plate, apparently designed to support the curious series of ampullated mucous 
canals on the eyeless side of the head of this fish (see Plate XXXI. figs. 6, 7). The two 
frontal bones are represented in Plate XXIX. fig. 15. 
In the Plaice (Platessa vulgaris) the general form of the cranium is much the same 
as in the Halibut ; but some of the asymmetries indicated in the latter have run to a 
much greater excess. 
The keel, on the under surface of the cranium (Plate XXIX. fig. 13), is strongly bent 
towards the eyeless side,! and its anterior extremity is also twisted strongly upwards on 
its long axis towards the same side. 
The external angular process of the frontal of the eyeless side (Plate XXIX. fig. 142) 
is similar to that in the Halibut ; but the interocular process (m) is almost completely 
non-developed, so that the greater part of the lower or external boundary of the orbit 
is formed by the frontal of the ocular side; and this cireumstance might easily lead a 
superficial observer to imagine that the interocular process of the bone of the ocular 
side is homologous with the external angular of thé opposite one. This seems to have 
been Rosenthal's idea when he speaks of the upper eye being “placed between Me 
two long processes of the frontal bones, after the manner of Cyclopian monstrosities pr 
But the untenableness of this idea will be at once apparent if we refer to the series of 
frontal bones figured in Plate XXIX. figs. 7, 11, 14, 15, and to the relations of the inter- 
ocular fibrous septum and olfactory nerves, which here occupy an exactly similar position 
to what they do in the Turbot (p. 269). 
The prefrontals (w&u) are fashioned much as in the Halibut; that of the ocular side 1s 
Pushed forwards somewhat in advance of the other (Plate XXIX. fig. 12). 
The nasal bone fi pital wall; anteriorly it is much more de- 
ne forms a large part of the orbi han 
veloped on the side e i ide of the he 
à orresponding to the eyeless side 0 : : 
anterior surface, on which the intermaxillary cartilage glides, 15 very obliquely ui 
me the eyeless side (Plate XXIX. fig. 12, Plate XXX. fig. 7). Compare 
the direction of the analogous in the Turbot (p. 271). 
gous part in the P à; 
Vomer, —The two facets ( Pen Plate XXIX. fig. 12) on the end of this ye "e E 
Which the heads of the superior maxillary bones glide, are SO placed that the T bo 
ng the angle which they form with each other passes very obliquely to the prenant 
* Loc. cit. 
