of the Fishery Board for Scotland. 



275 



nasal cartilage is comparatively thin but solid. The cartilage does not 

 pass down between the eyes, the smaller (upper) portion of the interorbitai 

 septum being bony (orbito sphenoidal), the main part toughly membranous, 

 with only the thread of cartilage already described, running along part of 

 its lower edge. 



The continuous cartilaginous skull in the adult herring is therefore 

 almost entirely anterior to the brain case. 



The bony skull, viewed from behind (PI. XVIII. fig. 23), is of a somewhat 

 square shape, with a pair of laterally projecting wings on its lower border, 

 the upper surface being formed by the supraoccipital (s.oc.) and upper 

 edges of the epiotic (ep.) bones, the lower narrower by the basioccipital 

 (b.oc.) and exoccipitals (ex.o.), while the prominent pterotics form the 

 lateral projections. The foramen magnum (f.m.), about equal in size to 

 the excavated articulating end of the basioccipital, is entirely formed by 

 the exoccipitals. These meet in the middle line along the upper surface of 

 the basioccipital. The whole posterior end of the basioccipital forms the 

 articulating concave condyle for the first vertebra. This posterior concave 

 surface is very slightly arched and hollowed out on its under edge, while 

 above there are two backwardly projecting small facets which overlap the 

 first vertebra, and have between them a triangular-shaped excavation for 

 the reception of a corresponding process, already described, on the first 

 vertebra. The notochordal remnant is almost entirely obliterated, although 

 in the young 40 to 60 mm. herring it can be traced forwards through 

 two-thirds of the length of the basioccipital as a gradually narrowing pro- 

 cess. Immediately in front of the articulating concavity, the basioccipital 

 takes the form in cross section of a narrow solid bar (PI. XVIII. fig. 24, b.oc), 

 which forms its dorsal edge, and from which pass downwards a pair of 

 strong lateral wings. These pass slightly outwards at first and then 

 inwards, the angle thus formed contributing on each side of the bone's 

 external surface a median lateral riclge, the posterior portion of which 

 meets with the exoccipitals. The basioccipital, therefore, viewed from 

 below, appears to form along nearly its whole length a channel, the con- 

 siderable depth of which is much increased by the wings of the parasphenoid 

 (PI. XVIL fig. 18; PI. XVIII. fig. 24, b.sph.w.). In this channel lie the 

 straight eye- muscles, and except that the wings of the parasphenoid 

 approach each other and partially close it, it is open beneath. There is no 

 floor formed to it by the parasphenoid, the median portion of which 

 scarcely extends behind the commencement of the prootics. 



Above and on each side of the basioccipital, and surrounding the whole 

 foramen magnum, are the exoccipitals. Appearing posteriorly, and on each 

 side of the middle line with a somewhat flattened surface, they send out and 

 upwards from their lower outer posterior faces a strong rounded projection 

 which meets on its upper surface with the lower part of the epiotic, and 

 on its lateral portion with the pterotic bones. The lower part of this 

 portion is excavated for the posterior end of the horizontal and the ampulla 

 of the posterior vertical semicircular canals. From its lower inner part a 

 process projects inwards and slightly backwards to be applied to the outer 

 side of the condylar part of the basioccipital. This strong process is con- 

 tinued forwards on its lower side to meet with the posterior edge of the 

 prootic. Above (but separated by a fenestra to be described later) the 

 junction of that bone with the basioccipital, and partly filling the space 

 between the bases of these two processes of the exoccipital, is a thinner 

 portion which meets its fellow of the opposite side in the middle, and 

 articulates with the dorsal surface of the basioccipital median ridge. 

 They thus form a floor for the medulla, and shut out the basioccipital 

 from any part in this. They also partially roof over the deep 



