PYCHODONTIDiE. 
193 
and these tesserae frequently fuse together. One median plate thus 
formed extends backwards between the anterior ends of the frontals, 
and these elements are sometimes entirely separated by it ; moreover, 
this median series is even continued further back by another azygous 
plate ( m.eth .) which meets (or is fused with) the supraoccipital. The 
arrangement of azygous median plates in the cranial roof of Mesturus 
leedsi is thus suggestive of the corresponding plates in Acipenseroid 
fishes. In Coccodus all the elements of the cranial roof are fused 
into one continuous shield. 
The cheek-plates are small, thin, and apparently without any 
very definite arrangement. In Mesturus and Gyrodus one plate 
larger than the remainder, bounding the upper jaw, may be the 
maxilla ; but it is delicate and toothless. The premaxillse are a 
pair of small, deep and narrow bones, moderately thick, resting 
upon the anterior extremity of the vomer, and covered with the 
ethmoidal dermal plates. The postero-external border of each is 
notched, as if for the opening of the nasal capsule. The sclerotic 
capsule is often ossified. 
The hyomandibular is a long narrow lamina about twice as 
broad above as in its lower half, with a posterior process near its 
upper end for the support of the operculum (PI. XYI. fig. 3, Jim.) ; 
but nothing is known of its connections below. The pterygo-palatine 
arcade is delicate and toothless, and is shown both in Anomoeodus 
willetti and Mesturus leedsi to have been directly fused for a 
considerable distance with the edge of the basicranial axis. The 
thickened articular end of the quadrate is turned forwards so that 
the facette for the mandible is in a vertical plane; and this 
facette is irregularly oval, deeper than broad, and slightly concave. 
The free border of the ectopterygoid is concavely arched; and 
there is not much doubt that a distinct symplectic element was 
apposed to the hinder margin of the quadrate, though this still 
remains to be definitely proved. The two rami of the mandible are 
separate at the symphysis, and in Mesturus leedsi each is observed 
to consist of five elements. The proximal end of the meckelian 
cartilage is distinctly ossified as an articular bone; and there is a 
very large splenial element within, while three more delicate splints 
' appear without. The splenial is much the largest and stoutest 
bone in the jaw, meeting its fellow of the opposite side at the 
symphysis and well covered with a pavement of teeth on its oral 
face. It is exposed on the outer aspect of the entire margin of the 
ramus behind the short oral border of the dentary, and rises pos¬ 
teriorly into a large coronoid process. The dentary is comparatively 
insignificant, thin, deep and narrow, meeting its fellow at the 
PART III. 
0 
