130 
ACTINOPTEKYGIl. 
prefrontal and ethmoid, there being no elongation of this foramen 
into a canal; and Dcipedius thus differs from Amia while agreeing 
with Lepidosteus in the course of the olfactory nerves across the 
orbital cavity. The membrane-bones of the cranial roof form a 
continuous shield, without vacuities ; but a narrow rim of the 
cranium projects behind the covering thus formed. In young 
individuals the paired parietals, frontals, and squamosals are dis¬ 
tinct, and this condition sometimes persists in the adult; but more 
usually these principal roof-bones soon become fused into a con¬ 
tinuous plate. Immediately in advance a pair of short and broad 
nasals meets in the middle line ; and there is a small median plate 
enveloping the ethmoid in front. The premaxillse are separate, 
small and short, apparently without any ascending process ; the 
maxilla is a long stout bone, with a slight spatulate expansion 
posteriorly, and bounded above in its hinder two-thirds by a small, 
narrow supramaxilla. A complete ring of circumorbital plates 
surrounds the eye; and there is also an incomplete suborbital series, 
reaching from the squamosal to the hinder end of the maxilla, while 
one isolated plate occurs immediately below the nasal opening, above 
the suture between the maxilla and premaxilla. The parasphenoid 
is imperfectly known, but does not appear to have been dentigerous; 
the vomer, which is probably single, is furnished with a dense 
cluster of comparatively large teeth. The hyomandibular is elon¬ 
gated and much laterally compressed, with a considerable expansion 
above, but contracted in its lower half into a narrow bar with nearly 
parallel anterior and posterior edges. The symplectic and quadrate 
are unknown, and the palato-pterygoid arcade can only be described 
as delicate, with large teeth on its inferior margin, smaller tuber¬ 
cular teeth on its inner face. The mandible is remarkably short 
and deep, with a coronoid elevation, comprising distinct dentary, 
splenial, articular, and probably coronoid elements. The dentary 
bears only the marginal series of teeth, and its postero-superior 
portion is smooth and toothless where overlapped by the maxilla ; 
the splenial is less deep, but more robust anteriorly, entering the 
mandibular symphysis and bearing a cluster of teeth, which become 
minute granules behind as the bone rises into the coronoid. The 
angular bone is deep, fixed to the outer face of the articular, and 
uniting with the dentary in a conspicuously zig-zag suture. Not¬ 
withstanding the prominence of the superficial ornament, the course 
of the sensory canals can scarcely be traced ; but a groove for the 
transverse commissure on the parietals and squamosals is seen, and 
a nearly vertical groove occurs on the angular bone. 
The opercular apparatus is complete, but the preoperculum ^.s 
