280 Appendices to Fifth Annual Report 



show two smaller apertures; — an anterior formed mainly by the prootic 

 and pterotic bones, and containing the ampullae of the horizontal and 

 anterior vertical semicircular canals lying between the two spherical air 

 vesicles, — the posterior formed, by the exoccipitals, contains the ampulla 

 (PI. XVIII. fig. 24, am.) of the posterior vertical canal. These two cavities 

 are separated mainly by the bone surrounding the anterior spherical air 

 vesicle. While the posterior vertical canal runs in the exoccipital, epiotic 

 and supraoccipital, the horizontal in the pterotic encircling and generally 

 in a groove around the outer surface of the smaller air vesicle, the anterior 

 vertical is enclosed only by the cartilage internal to the pro- and sphenotics, 

 and just within the foramen between these two, which is covered externally 

 by the lateral wing of the frontal. The vertical part common to the 

 anterior and posterior canals is not surrounded by bone or cartilage, but lies 

 free in the brain cavity, although close to its otic wall.* 



Of the membrane bones of the roof of the skull, the frontals are by far 

 the largest. They are long bones extending from above the centre of the 

 nasal cavities nearly the whole length of the head, terminating close to the 

 supraoccipital, from which (except its median piocess) they are separated 

 only by the small parietals. Widest over the centre of the orbital 

 region, their outer edge is there turned slightly downwards and inwards 

 (figs. 18, 25, /r.), overlapping the edge of the cartilaginous tegmen. They 

 each terminate anteriorly in a pointed end overlapping the posterior pro- 

 cesses of the ethmoid. Along their inner margins they are closely approxi- 

 mated from their posterior ends to a point just in front of the fore-brain, 

 where they separate. The suture between them is nearly straight, except 

 for a small interlocking zigzag portion over the fore brain, where the 

 substance of the bone is about its thinnest. Anterior to the fore-brain 

 then the separated frontals leave the cartilage uncovered except by skin. 

 The thin part of the bones over the fore-brain is raised slightly as a central 

 lozenge-shaped portion, posterior to which they slope slightly downwards 

 from their outer margins towards the middle line — forming a hollow on 

 the roof of the skull, filled with gelatinous tissue, slight in the herring, 

 but much more pronounced in the pilchard. In line with the thalamen- 

 cephaion, each frontal sends down from its outer edge a long relatively 

 wide ridged process (fig. 18, fr.pr.), the posterior margin of which over- 

 laps the anterior edge of the pterotic, while its anterior edge is covered by 

 the posterior margin of the sphenotic. This lateral frontal process thus 

 closes in the elongated aperture already described as occurring between 

 the pterotic and sphenotic of each side. 



Behind, and having their anterior borders overlapped by the frontals, 

 and lying between the latter and the transverse ridge on the dorsal surface 

 of the supraoccipital, are the small parietals (par.). These are triangular- 

 shaped above, one-half being overlapped by the frontals, while the other 

 moiety projects backwards over the outer surface of the epiotics, the apex 

 of the triangle facing inwards, and meeting with the supraoccipital. 

 The median process of the supraoccipital separates the parietals from one 

 another. About the centre of their under surface there arises a process, 

 which extends outwards and downwards, moulded in the cartilage within, 

 and with its bifurcated tip terminating beneath the upper part of the 

 pterotic. This process forms the anterior boundary of the postero-lateral 

 cavity and the posterior margin of the foramen (for). In front of the 

 parietal process occurs a large semicircular foramen (PI. XVII. figs. 18, 

 19, for.) opening directly into the brain cavity. This foramen is occasioned 

 by the downwardly hollowed upper edge of the cartilage and pterotic 



* Most of the details of the herring's ear and its swim-bladder connections were 

 most accurately described by .Weber, De Aure et Auditu Hominis et Animalium, 

 pars, i., Lipsise, 1820. 



