PTCNODONTID.E. 
191 
with the median ridge on the superior aspect of the coalesced vomers : 
it can be distinguished from this plate by the arrangement of its 
structural fibres, which are directed downwards and forwards 
instead of horizontally, and it never projects in advance of the 
vomers in the rostrum. There is no ossified interorbital septum. 
It is evident from several specimens, notably Anomoeodus willetti 
and Mesturus leedsi, that the base of the cranium bears a delicate, 
laminar, median vertical keel extending as far forwards as the orbit; 
but the only precise information concerning the basicranial axis has 
been obtained from specimens of Mesturus leedsi. The basiocci- 
pital is unknown, but a detached element, evidently chondrocranial, 
in no. P. 6834 recorded below, fits well on the hinder margin of 
another bone in the same fossil which is proved to be parasphenoid. 
It may therefore be determined as basisphenoid, and is noteworthy 
as occupying a considerable length of the basicranial axis. It is 
longer than broad, exhibits a median longitudinal keel below, and 
is narrowest at its hinder end; while the lateral margins form 
sharp edges, each apparently .notched near its anterior end, and a 
pair of large foramina pierce the bone obliquely near its middle. 
The parasphenoid is remarkable for its shortness, not extending 
backwards beyond the front edge of the supposed basisphenoid. 
The main part of the bone is short, stout, and broad, with a median 
longitudinal keel below ; and its basipterygoid processes are short, 
with the very coarsely dentated articular end already mentioned. 
At the front extremity of the otic region this broad bone is abruptly 
truncated, its inferior keel terminates in a roughened pointed knob, 
and the element continues merely as a narrow slender median bar 
across the region of the orbit; towards the front of the orbit in 
many fossils a vertical median lamina is observed to rise gradually 
on its superior face to meet the ascending plate of the coalesced 
vomers. The latter form one very stout, bilaterally symmetrical 
bone extending to the anterior end of the rostrum, where they 
are overlapped by the small premaxillae. 
The arrangement of the membrane-bones (or dermal plates) of 
the cranial roof is not quite constant, and there is much irregularity 
especially in the superficial elements covering the ethmoidal region. 
As shown by Mesturus (fig. 30), the posterior median element of the 
roof is bilaterally symmetrical and unpaired, and may therefore 
be named supraoccipital ( s.occ .). The frontals (fr.) are the largest 
plates, always conspicuous and readily identified between the orbits, 
and varying in length according to the shape of the head. The 
oiic region is covered by a single plate, which forms the postero¬ 
lateral angle of the roof and meets the frontal anteriorlv. entering 
