184 
Discussion of the Asymmetry. 
We have now ascertained the facts which justify a 
fuller discussion of the causes and course of the asymmetry 
of the head. ‘To satisfactorily understand the considera- 
tions which we put forward, the reader will find it essen- 
tial to follow our argument with dried crania both of a 
Plaice and Cod before him. We have already described 
the cranium in detail, but it will be useful to summarize 
those features relevant to the present discussion. 
Note then that (a), the bony interorbital septum is 
formed by the right frontal alone, that it is a thin flattened 
bone lying in the morphological horizontal plane of the 
head, and that in front it is in contact with the ethmoid 
cartilage which is perforated by a wide fenestra; (8), that 
the left prefrontal has lost its primitive connection with 
the left frontal* but is still attached to the parasphenoid 
bar, while the right prefrontal, though still in contact 
with the right frontal has lost its connection with the 
parasphenoid bar, and is further distinctly anterior to the 
left bone; (c) that all the oblique muscles take origin from 
the left prefrontal. 
Note now the differences between these relationships 
and those of the corresponding structures in the Cod’s 
skull. In the latter (a), the fused frontals form a broad 
roof to the cranium, and internally and below two bony 
ridges form the bony portion of the interorbital septum, 
whilst the ethmoid cartilage is not perforated; (B), the 
right and left prefrontals are in contact with the frontals 
of their own side and with the parasphenoid bar; (c), the 
oblique muscles take origin from the upper portion of a 
stvong membranous partition joining the frontal ridges 
* The present connection between the left frontal and left prefrontal is 
purely secondary, as mentioned elsewhere. 
