of the Fishery Board for Scotland. 



277 



a concavity in the pterotic a deep hollow, overhung by the upper limb of 

 the epiotic, and open on the postero-lateral aspect of the cranium (PI. 

 XVII. fig. 18, cav.). Around .and within this cavity, most of which is 

 roofed by the parietals, are attached some of the dorsal muscles. The 

 bone forming its anterior edge encloses part of the horizontal semicircular 

 canal, most of which, however, is surrounded by the cartilage internal to 

 this part. The ossification over it, and just behind the supraoccipital lateral 

 ridges, is sometimes so slight as to scarcely withstand the removal of the 

 skin. A portion of its inner anterior end is, except in the larger herrings, 

 thin and cartilaginous, the cartilage of the auditory region here persisting 

 on the inside of the epiotic and pterotic ossifications. This cartilage has 

 been described as appearing in shape of a narrow band posteriorly between 

 the exoccipitals and supraoccipital, and enclosed within the portion internal 

 to the anterior upwardly projecting part of the pterotic is the anterior 

 semicircular canal. Viewed from the side the cranium presents a quadri- 

 lateral form, hollowed on its upper edge when the frontals are removed, 

 and is formed by the sphenotic, pterotic, and epiotic bones. The two 

 lower corners terminate in a sharp process, the anterior of which belongs 

 to the sphenotic which forms the anterior outer edge of the otic. mass. 

 This sphenotic is somewhat triangular in shape, the anterior basal portion 

 curving outwards and downwards to form the sharp process just referred 

 to; w T hile the posterior border slopes slightly backwards and upwards, 

 forming a sharp edge on its outer surface, separated from a similar posterior 

 edge of the pterotic by a narrow elongated aperture (PI. XVIII. fig. 22, gr.), 

 which is covered externally by a process of the frontal bone (PI. XVII. 

 fig. 18, fr.pr.). On its inner ventral border the sphenotic meets the prootic. 

 Immediately external to this junction, and therefore directly under the 

 outer overhanging margin of the sphenotic is a groove for the articula- 

 tion of the anterior part of the head of the hyomandibular (PL XVII. 

 figs. 18, 21, ar.hy.). Joining posteriorly as the sphenotic does with the 

 pterotic, this articular cavity is continued along the under outer side of 

 the latter also, with which indeed the longer portion of the hyomandibular 

 head articulates (PI. XV. fig. 1). This articular hollow in the pterotic ends 

 under a backwardly and slightly downward projecting sharp process for 

 muscle attachment, just internal to which is the connection with the ex- 

 occipital and epiotic. It is the posterior part of the epiotic terminating 

 in this process which forms the outstanding lateral wings referred to as 

 seen in a posterior view of the skull. 



On its inner ventral surface the pterotic meets the small opisthotic 

 (PI. XVII. fig. 21, op.ot.) and the outer posterior border of the prootic. 

 Viewed from below, and projecting from that part of the pterotic bounded 

 internally by the prootic and opisthotic, and externally by the hyoman- 

 dibular articular cavity, there is seen a segment of the smaller and posterior 

 of the two spherical air vesicles in connection with the swim-bladder 

 (figs. 18, 21, p.s.va.). The larger segment of this capsule is seen from the 

 side forming the lower part of the cavity referred to above as largely formed 

 by the epiotic bone, and it is closely encircled by the membranous horizontal 



semicircular canal (PL XVIII. fig. 24, p.s.vec.h.sc.c). The small opisthotic 



between the pterotic, lower aspect of exoccipital and prootic — partly covers 

 the point of junction where the single duct from the swim-bladder, after 

 passing through the pear-shaped air cavity, branches into two canals, one 

 leading to each spherical capsule. 



The sphenotic and pterotic overhang the prootic and opisthotic, and face 

 so directly outwards that, as a consequence, the real outer surface of the 

 prootic looks almost directly downwards. The prootic articulates on its 

 outer upper edge with the sphenotic and pterotic, posteriorly with the 



