330 WOODWARD : FOSSIL FISHES OF THE UPPER LIAS OF WHITBY. 
therefore, the preoperculum does not appear to reach the cranial 
roof when the cheek-plates are all in position, it really completes 
that extension beneath the upper suborbitals. The operculum (op.) 
and suboperculum (s. op.) are similar to those of the last specimen, 
and there are also fractured remains of an interoperculum (i. op.) of 
corresponding proportions. The external surface of these bones is 
not well preserved, but it may be noted that there are no traces of 
a tubercular ornament. The only doubtful indications of branchi- 
ostegal rays are some fragments at the hinder end of the right 
mandibular ramus. The head is crushed upon the remains of the 
trunk in such a manner that the clavicular arch and post-clavicular 
plates are obscured ; but some of the anterior series of scales are 
very well exhibited. The median dorsal ridge-scales are shown to 
be at least as small as the adjoining upper flank-scales, and they are 
not produced into a conspicuous point or spine behind. The exposed 
enamelled portion of the principal flank-scales is deeper than broad, 
and the hinder border is serrated in its lower half or lower two- 
thirds, the acute denticles being directed obliquely downwards and 
apparently continued as short feeble ridges on the bone beneath the 
superficial enamel. The smaller upper and lower flank-scales are not 
serrated, but their postero-inferior angle is produced into a slender 
point. The right pectoral fin is enveloped in a mass of hard matrix, 
on the surface of which the characteristic double row of fringing 
fulcra (pet.) is very well displayed. 
Another more imperfect head in the British Museum 
(No. P. 52 13) of the same size as the last, also shows the coarse 
tuberculation of the cranial roof and the cheek-plates ; but the 
operculum and suboperculum still remain smooth. The most 
interesting feature of this specimen, however, is the preservation of 
the large, deepened posterior suborbital (pi. xlvi,, fig. 2), which 
exhibits three distinct surfaces of overlap (1, 2, 3) for circumorbitals 
in front. 
Still another larger specimen (Brit. Mus., No. P. 74 10) has the 
coarsely-tuberculated and rugose cranial roof and cheek-plates, so 
that the comparative smoothness of these bones in the first skull 
described is evidently a mark of immaturity. The operculum and 
