STKUCTUEE OF CERTAIN PALiEOZOIC DIPNOI. 
173 
edge. At the back, however, this weak symphysis is supported on. the 
cranial face for some part of its length by the anterior process of the 
parasphenoid, though that still leaves the forward part and the long anterior 
ridges of the tooth-plates to all appearance very insufficiently provided 
against the strains of mastication. This region, however, no doubt received 
further support from the overlying cartilaginous cranium. 
Fig. 8. 
A, Sagenoclus ; B, Ctenoclus. Left pterygoid and tooth-plate, X f. 
As they occur in the shales, the pterygoids and their tooth-plates are 
nearly always crushed into one plane. There can be no doubt, however, 
that in life the wing of the pterygoid would be bent downward along its 
outer edge to support the quadrate. It may not have been bent so sharply 
as in Ceratodus, where it stands at right-angles to the tooth-plate, but that 
it was considerably bent is proved by the abundant cracks that break the 
