278 



Appendices to Fifth Annual Report 



opisthotic and exoccipital, and on its posterior inner border with the basi- 

 occipital. Conspicuous on both its external and internal (to the brain 

 cavity) surfaces is the bony wall of the larger spherical air vesicle (PI. XVII. 

 figs. 18, 21, a.s.ves.). A small part of the posterior portion of the prootic 

 forms the apex of a triangular-shaped fenestra (figs. 18, 21, 24, au.f.), in 

 the side, or rather iu what, from its form, must be called the base of the 

 herring's skull. Of the two longest sides of this foramen the outer is 

 formed, as already mentioned, by part of the exoccipital, and is overhung 

 by the opisthotic, and the inner (ventral) side by the outer edge of the 

 basioccipital. Its base or posterior end is bounded by the descending part 

 of the exoccipital which contains the swim-bladder duct. This fenestra is 

 surrounded by connective tissue and muscle immediately dorsal to the 

 pharyngobranchials, and is covered by a delicate tense semi-transparent 

 membrane, within which, and closely applied to it, is the sacculus of the 

 ear with its contained large otolith. This can be readily seen shining 

 through the membrane, which almost bears the appearance of a tympanum. 

 It seems to be the persistent homologue of the fenestra found in the 

 young salmon.* 



The anterior edge of the prootic forms the anterior boundary of the 

 cranium, and its inner edge meets with that of its fellow of the opposite 

 side in the middle line (PI. XVII. fig. 21). The median portion of the 

 prootics (i.e., on each side of the suture), and about 2 mm. in width from 

 before backwards, forms a flat shelf on which rests the pituitary body 

 (PI. XVII. fig. 19, pt.f.). This is let down behind a foramen in the anterior 

 wall of the cranium formed by the transverse portion of the basisphenoid 

 and this part of the prootic. Behind the pituitary body the prootics form 

 an upstanding transverse ridge, rounded on the upper surface, and passing 

 across like a rampart between the right and left large spherical air vesicles 

 (PI. XVII. fig. 19, pr. ot.). The ventral outer edges of the prootics project- 

 ing downwards are continuous with the ventral wings of the basioccipital, 

 and form the sides — as the median part forms the flat roof — of the anterior 

 moiety of the eye-muscle canal. 



Articulating with the anterior inner corners of the prootics, and forming 

 a bar across the anterior portion of the brain case, is a narrow bone. From 

 its centre there projects downwards a relatively long but very delicate needle- 

 like process, which partially separates the right and left recti muscles of 

 the eye, where they pass backwards into the eye-muscle canal. This is 

 the basisphenoid (figs. 19, 21, 25, b.sph.) ; the transverse part seems to 

 correspond to the V-shaped portion and the spine to the vertical arm of the 

 basisphenoid of the salmon and some other teleosts. The delicate vertical 

 part generally arises from the stronger transverse part in a small bifurcated 

 base, so small, however, as often to appear a single stem. Its lower tip is 

 not applied to the parasphenoid, as in the salmon, but is only attached to 

 the interorbital septum. Perforating each end of the transverse part of 

 the basisphenoid are the foramina for exit of the oculo-motor nerves of 

 the eye muscles. 



From the upper anterior margin of the prootics there stand up a pair of 

 nearly circular (in outline) flattened alisphenoids (at.), perforated near their 

 centre for the abducent nerve to the superior oblique muscle. To their 

 lower anterior corners are also articulated the end of the basisphenoidal arms 

 above described. Articulating with the upper part of the alisphenoids is 

 a hollowed or V-shaped orbitosphenoidal (PI. XVII. figs. 18, 19, or.spM.) 

 ossification, in which lies the fore-brain. Anteriorly the upper end of the 

 limbs bend inwards to nearly meet in the middle line, while the lower angle 

 is continued forwards as a median thin irregular ossification as far as the 

 " The Salmon's Skull," by J. S. Parker, Mil Trans.. 1878. 



