CTENOTHRISSA. 81 



circumorbital ring resemble those of the trunk, being deeply overlapping, rugose, 

 and serrated. 



The opercular apparatus is narrow and deep, with a finely rugose ornament, and 

 very little of it, if any, can have been covered with scales. The preoperculum 

 (PI. XVII, figs. 1, 2, pop.) is sharply bent at its angle, and the lower limb is about 

 two-thirds as long as the ascending limb. It is much expanded at the angle, and 

 the ascending limb tapers more rapidly than the lower limb. It is traversed by a 

 very deep groove for the slime-canal, and the bone of its lower limb is coarsely 

 crimped as well as rugose. The operculum (PL XVII, figs. 1, 2, op.) is trapezoidal 

 in shape, narrowest above and widest at the lower end, with a slightly wavy hinder 

 edge. The length of its lower edge equals nearly two-thirds that of its anterior 

 edge. Its ornamental rugae radiate backwards and downwards from the point of 

 suspension, and are absent on a narrow antero-superior area, which seems to have 

 been covered with scales, though these have not been observed. The suboperculum, 

 well seen in the type specimen and in the original of PI. XVII, fig. 1 (sop.), is 

 triangular in shape, its maximum depth at the front edge equalling half its length. 

 Its antero-superior angle is produced into a short ascending peg. Its external 

 ornament is confined to the anterior half of the bone, but no scales have hitherto 

 been noticed over the hinder smooth area. The interoperculum (PI. XVII, figs. 

 1, 2, lop.) is relatively large and extended, with its delicate rugose ornament chiefly 

 disposed in lines at right-angles to the lower edge. The number of branchiostegal 

 rays is uncertain, but there cannot have been less than eight, perhaps ten. Those 

 attached to the epihyal (PL XVIII, fig. 2, hr.) are broad, rapidly tapering, curved 

 laminae ; those fixed to the middle and front half of the ceratoLyal (PL XVIII, 

 fig. 3, br.) are smaller, thinner, and more sharply bent. 



The vertebral column is imperfectly known, but the specimen represented in 

 PL XVIII, fig. 1, shows that the vertebras are small and at least 35, perhaps as 

 many as 40 in number. The centra are about as long as deep, and strengthened 

 with fine longitudinal ridges. The ribs are stout and extend almost or quite to the 

 ventral border. The neural and hamial arches in the caudal region are fused with 

 the anterior part of each centrum. 



A single pair of transversely extended supratemporal plates overlaps the 

 occiput. Each of these bones (PL XVII, fig. 1, st.) is nearly triangular in shape, 

 with the apex directed to meet its fellow of the opposite side in the median line. A 

 transverse slime-canal must have occupied a deep groove in its outer face, which is 

 coarsely rugose and crimped. The post-temporal (PL XVII, fig. 1, ptt.) is a thin 

 lamina of bone, probably about as deep as broad, having a rounded hinder edge and 

 the posterior half ornamented with delicate, radiating rugae. The supraclavicle 

 (PL XVII, fig. 2, scl.) is a narrow lamina, about four times as deep as wide, of 

 nearly uniform width except where it tapers at the lower end to overlap the clavicle. 

 Its outer face is almost smooth. The clavicle (PL XVII, fig. 5, cl. ; PL XVIII, 



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