Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. loi 



The postero-inferior portion of the supraoccipital is narrow, and 

 is pointed at its lower end. It Hes in the middle line of the pos- 

 terior surface of the skull, and overlaps externally the adjoining 

 edges of the exoccipitalia and occipitalia lateralia. 



Along the middle line of the dorsal portion of the bone there is 

 a strong, longitudinal ridge, which separates the supratemporal 

 groove of one side of the head from that of the other side. This 

 ridge is continued downward along the middle line of the poste- 

 rior portion of the supraoccipital, and the entire ridge, so formed, 

 is produced posteriorly into a long, thin, spina occipitalis. This 

 spina is usually triangular in lateral outline, and its dorsal edge, 

 which lies in the general level of the dorsal surface of the skull 

 and head, ends in a sharp point directed backward and upward. 



The supraoccipital takes no part whatever in the formation of 

 the enclosed portions of the bony semicircular canals, but that part 

 of the edge of its dorsal portion that adjoins the exoccipitale, 

 forms, on the inside of the skull, a part of the mesial wall of the 

 cerebral groove that lodges the unenclosed portion of the posterior 

 membranous semi-circular canal, and, in front of the dorsal end 

 of that canal, the anterior membranous canal. 



The Basioccipital (BO) is about twice as long as it is wide 

 and thick. Its hind end is solid and vertebra-like in shape, and 

 of somewhat greater diameter than that part of the bone that lies 

 immediately in front of it. The dorsal surface of this vertebra- 

 like hind end of the bone is much longer than its ventral surface, 

 the latter being reduced to little more than a narrow transverse 

 edge. Its posterior surface, which is also the posterior surface 

 of the basioccipital itself, is entirely occupied by a deep conical 

 depression, a narrow outer edge of the hind end of the bone alone 

 remaining. The depth of the depression, in some specimens, al- 

 most equals its diameter. A similar depression is found on the 

 anterior end of the first vertebra, the two depressions enclosing a 

 large bi-conical space, which is filled with a soft and somewhat 

 gelatinous mass. 



Immediately in front of its solid, vertebra-like hind end the 

 basioccipital is contracted laterally and dorsally, but not ventrally. 

 It then expands gradually to its anterior end, which is somewhat 

 larger in diameter than its solid hind end. This part of the bone 



