THE SKULLS OF FISHES AND AMPHIBIA. 211 



4. The opisthotic bone, occasionally absent as a distinct 

 ossification, is very small in some fishes, such as the Perch (where 

 it is Cuvier's "rocher" or "petrosal"), but becomes very well 

 developed in such genera as Ephippus, and attains an immense 

 size in the Gadidee. 



5. The canal for the orbital muscles is absent in many fishes, 

 such as the Cod tribe. 



6. The most remarkable modification of the fish's cranium 

 proper, how T ever, is the want of symmetry produced in the flat 

 fishes, or Pleuronectidse, by a sort of twist, which affects the 

 anterior and upper, but not the hinder and inferior, part of the 

 skull. Thus, if the skull of a Turbot be examined, the supra- 

 occipital w r ill be found in its ordinary place ; while the epiotics 

 and squamosals are symmetrically disposed on each side of it, so 

 that the skull, viewed from behind, is like that of any other 

 ordinary osseous fish. The basi-occipital, parasphenoid, and 

 vomer are likewise arranged, as usual, along the median basal 

 axis of the skull. The pro-otics and post-frontals are also nearly 

 symmetrical, but the alisphenoids are thrown over to the left 

 side, so that the anterior aperture of the cranial cavity, be- 

 tween the alisphenoids, lies no longer immediately over the 

 parasphenoid, but to the left of it. The left frontal sends down 

 a long curved process, which joins with one from the prefrontal 

 of the same side, and the two eyes come to lie in the secondary 

 orbit, developed between the curved bony boundary thus formed 

 and the median frontal crest. 



7. An addition takes place to the posterior extremity of the 

 skull, in many fishes, by the anchylosis with it, and with one 

 another, of a variable number of vertebra?. Cartilaginous verte- 

 bra), as I have already pointed out, coalesce with the cartila- 

 ginous skull in both Accipenser and Spatularia, and two or three 

 bony vertebra? are anchylosed with the osseous skull in Lejndos- 

 teus and Pohjpterus. Whether a similar addition takes place in 

 the other living ganoid, Amia, or not, I cannot say. In many 

 Siluroids a great number of vertebra? become thus anchylosed 

 with one another and with the skull. 



8. In both Siluroids and Ganoids, again, an addition to the 

 roof of the skull is effected bv the coalescence therewith of the 



p 2 



