37 



and the basi-occipital is a small square bone, the opisthotic^ op.o., and in front of this 

 below the sphenotic is the prootic, pr.o. 



Adjoining the edges of both prootics and sphenotics, and attached behind to the 

 basi-occipital, is an elongated keeled bone which projects forwards to form the ventral 

 process of the facial portion of the skull : this is the parasphenoid, pa.s ; its posterior 

 portion forms the anterior part of the floor of the cranium. Attached to the ventral 

 surface of the parasphenoid is a bone which is elongated dorsally and projects 

 ventrally into a cylindrical knob ; this is the vomer, vo. The interorbital septum is 

 formed by the anterior processes of two bones whose flat posterior portions are nearly 

 symmetrical and form the anterior part of the roof of the cranium, attached behind to 

 the edges of the supra-occipital and parietals. These are the frontal bones, r.f., l.f. 

 These bones are, as compared with the corresponding bones of the skull of a 

 symmetrical fish, the most distorted of all the bones in the skull of the sole. In a 

 symmetrical fish the frontal bones are perfectly symmetrical, and their anterior parts 

 lie above and between the symmetrically placed eyes. In the sole also their anterior 

 processes are between the orbits, but have been twisted round through an angle of 

 90, so that the middle line of the dorsal surface of the cranium when produced 

 forwards does not pass between the frontal bones, but a line drawn in a longitudinal 

 direction over the right surface of the cranium, across the pteroic and sphenotic bones 

 does pass between their anterior processes. This shows that the dorsal eye of the sole 

 is really the left eye, and that the eyes and orbits with the interorbital septum have, 

 as compared with those of a symmetrical fish, been twisted round through an angle of 

 90 while the cranial portion of the skull remained stationary. Connecting the 

 inferior process formed by the parasphenoid and vomer with the interorbital septum 

 anteriorly is a single bone having a somewhat hooked process in front the mesethmoid^ 

 mes.e. Attached to the mesethmoid posteriorly on the left side, and joined also to the 

 parasphenoid, is a large flat bone concave towards the right, the left ectethmoid^ ect.e. 

 A posterior process of this bone unites with an anterior process of the left sphenotic to 

 form the third process of the facial region of the skull. The right ectethmoid is a small 

 ring-shaped bone attached ventrally to the anterior end of the frontals and to the 

 mesethmoid. 



Between the origins of the three facial processes the cranial cavity opens anteriorly 

 by a wide aperture. There is a large round foramen in the left ectethmoid, leading 

 from the left orbit to the external surface of the facial part of the skull. On the upper 

 surface of the cranial portion there is a small foramen in the flat posterior portion of 

 each frontal bone, the left foramen being larger than the right. At the anterior and 

 inferior edge of each sphenotic bone there is a considerable indentation, which, with a 

 corresponding indentation in the edge of the parasphenoid, forms a large foramen, the 

 sphenotic foramen. That of the left side is twice as large as that of the right, and in 

 front of the former are two small foramina which are absent on the right side. 

 Through each prootic bone is a smaller foramen, the prootic foramen ; the left of these 



