woodward: fossil fishes of the upper lias of WHITBY. 467 
shape from that of Chondrosteus, and more closely resembles 
that of the modern sturgeon (fig. 6). It agrees with the latter 
in its arched form, expanded anterior end, and a well-developed 
expansion {x) for articulation with the palatine element; but it 
differs in the relatively greater steepness of its anterior border, 
and in the straightness, shortness, and posterior expansion of its 
hinder half. 
The slender hour-glass-shaped bone already mentioned as 
probably being ceratohyal is about as numerous as the hyoman- 
dibular, and two detached specimens in the British Museum 
are shown in the accompanying figs. 7, 8. They appear to be 
somewhat more slender than the corresponding element in 
Chondrosteus and the sturgeon. 
One nodule from the bed immediately above the jet rock, 
obtained from a Whitby dealer by the present writer in 1895, 
almost certainly exhibits the left opercular apparatus from the 
inner aspect, and is the only known specimen displaying this 
part of the skeleton. It is shown of one-third the natural 
size in pi. Ixix. An intractable mass of pyrites unfortunately 
obscures part of the fossil from the top of the suboperculum 
downwards, and there is a crack across the latter element near 
its lower end ; but the essential characters of the bones are 
observable. The operculum {oj?.) is a comparativel}^ small plate, 
truncated in front, and rounded postero - superiorly : it is 
strengthened by two nearly vertical but slightl}^ convergent 
ridges, which extend downwards from its point of suspension. 
The suboperculum (s.op.) is remarkably narrow and deep, its 
total depth being about three times as great as the maximum 
depth of the operculum. Its radiating structural fibres cause 
a slight fimbriation of its upper, lower, and hinder borders. 
Beneath this plate follow seven branchiostegal rays (br.^'') in 
natural order, all fimbriated at the truncated hinder border, 
but otherwise smooth on the inner face. These diminish in 
width downwards, so that the lowest is the most slender, but 
the diminution is b}'^ no means regular or gradual. If this 
fossil be correctly interpreted, it is therefore evident that 
