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Appendices to Fifth Annual Report 



ventrolateral cavities in which lie the membranous sacculi of the ears 

 (PI. XVIII. fig. 24, cav.sac). From its junction with the pterotic and 

 opisthotic, i.e., from its extreme lateral projection, there runs down the 

 lower posterior outer edge of the exoccipital to the outer edge of the 

 basioccipital condyle, a sharp-pointed process formed by a pair of ridges, 

 between which are the foramina for the 9th and 10th nerves (PI. XVII. 

 figs. 18, 21, ix. x.), while immediately above the termination of the upper 

 (outer) ridge, and between it and the side of the basioccipital condyle, 

 the tubular cartilaginous rod containing the membranous duct from the 

 swim-bladder passes into the bone, the perforation running slightly up- 

 wards within that thickened portion of the exoccipital which forms the 

 floor of the bony fold containing the posterior ampulla. The canal for 

 this duct is in the pilchard about J mm. in diameter, less in the herring, 

 and still less in the shad, where it passes from the exoccipital into the 

 pterotic, but the portion within the exoccipital expands in its course 

 into a pear-shaped vesicle — the posterior of the three well-known vesicles 

 in communication with the swim-bladder. 



Separated from the upper edges of the exoccipitals by a narrow strip of 

 cartilage (fig. 23) continuous with the portion which lies internal to the 

 otic bones, and occupying the upper central posterior end of the cranium, 

 is the supraoccipital. It is a comparatively thin bone, and in longitudinal 

 vertical section forms almost a right angle (PI. XVII. fig. 19, s.oc). The 

 posterior face is wide and somewhat triangular shaped, with a slight 

 median ridge and a rounded lateral ridge or rather fold on each side, the 

 ends of which meet the upper anterior ends of the epiotics, while they and 

 the median ridge originate in the middle dorsal line forming a very small 

 pointed backward projecting process, the only representative of an occipital 

 spine (fig. 23, sp.). The supraoccipital takes no part in forming the fora- 

 men magnum, being separated entirely from it by the exoccipitals. 

 From the base of the little spine above referred to, a slight dorsal ridge 

 passes outwards to the edge of the bone, and in front of this ridge the 

 bone takes a slightly depressed semicircular form with a short anteriorly 

 projecting median process which meets with the median strip of cartilage 

 (tr.car.) already described. Where it meets with the epiotics, the inner 

 edges (within the brain cavity) of each of the lateral folds above described 

 may be so approximated as to meet and fuse with each other so as to form 

 a short complete or partial canal (continuous with that in the epiotic) 

 containing the membranous posterior semicircular canal, where it passes 

 inwards at its upper end (PI. XVII. fig. 19; PI. XVIII. fig. 24, p.sc.c). 

 Sometimes this supraoccipital canal is not entirely closed by a complete 

 fusion of its sides, and sometimes it is closed only by the primitive cartilage. 



The most prominent bones on the posterior face of the cranium are the 

 epiotics which stand boldly outwards and backwards on each side. They 

 take almost the shape of the posterior semicircular canals, most part of 

 which is enclosed by them. Over the most prominent projection of the 

 curve thus formed the epiotics are thickened slightly, and form a sharp 

 angle. The outline of these rather flattened bones, therefore, forms an 

 obtuse angle externally, while internally or anteriorly (forming their junc- 

 tion with the ex- and supra-occipitals), they form the segment of a circle. 

 While the upper end of the projecting elbow 7 s which contain the semi- 

 circular canals and which are overlapped by the parietals, meet the supra- 

 occipital, the lower ends articulate with a projecting rounded process of 

 each exoccipital. The sharp angle above described as occurring on this 

 elbow forms the most backwardly projecting point of the cranium, slightly 

 exceeding that of the condyle. The portion of bone lying within the 

 segment formed by the canal is somewhat compressed, and forms with 



