328 WOODWARD : fossil fishes of the upper lias of WHITBY. 
lower circumorbitals, which are well exposed on the left side, exhibit 
the ornament only in the peripheral half, where it is somewhat 
radiating. The remains of the posterior suborbitals are quite 
smooth. The uppermost plate of this series, best shown on the left 
side, is in contact with the concave outer margin of the squamosal, 
but has not more than half the antero-posterior extent of the latter 
element ; it is quadrilateral, but deepest in front and gradually 
tapering towards the hinder border. The plate immediately below 
this is clearly deeper than broad, with slightly concave anterior and 
inferior margins. The remains of the upper jaw are too imperfect 
for description, but there are traces of stout, styliform upper teeth 
towards the end of the snout. The mandibular rami are sharply 
curved inwards at the symphysis, almost forming a horse-shoe bend 
(pi. xlvi, fig. la) ; and on both sides of the jaw the suture is distinct 
between the long dentary bone (d.) and the comparatively short and 
deep angular element (ag.). The preoperculum is shown on each side 
to be a narrow arched bone, apparently tapering to a point above at 
some distance beneath the cranial roof (see page 329), and not much 
expanded in its lower half, where it is marked by the usual longi- 
tudinal sensory canal, from which rugose markings radiate backwards. 
The other opercular plates are quite destitute of ornament. The 
operculum is narrowed upwards, and its maximum width on both 
sides is shown to be about two-thirds its depth. The maximum 
depth of the suboperculum is half its greatest width ; and a small 
process is directed upwards from its antero-superior angle. The 
interoperculum is a comparatively small triangular plate, antero- 
posteriorly elongated. No branchiostegal rays are preserved, but 
remains of the partly-displaced squamation extend forwards, even 
between the hinder ends of the mandible. The scales of this region 
are relatively small and very deeply overlapping. 
Another skull, with remains of the anterior part of the 
abdominal region (Brit. Mus., No. P. 3528a), is much fractured, but 
exhibits more of the cheek-plates and dentition than the last speci- 
men. It is shown of two-thirds the natural size from the right 
lateral aspect in pi. xlvii., fig. 1. The remains of the frontal 
parietal, and squamosal bones of the cranial roof exhibit a sparse 
