ASPIDORH YNCniDiE. 
415 
Suborder III. AETHEOSPONDYLL 
Notochord varying in persistence, but pleurocentra and hypo- 
centra usually fused, never forming alternating discs or rings ; tail 
abbreviate-heterocercal or homocercal. Mandible complex, with 
well-developed splenial rising into a coronoid process, which is 
completed by a distinct coronoid bone. Infraclavicular plates 
wanting in the pectoral arch ; pectoral fin with more than five 
basals. Scales ganoid. In the living forms—air-bladder connected 
with the oesophagus in the adult, optic nerves not decussating, but 
forming a chiasma, and intestine with remnants of a spiral valve. 
Synopsis of Families. 
A distinct presymphysial bone in man¬ 
dible ; vertebral centra annular or 
amphicoelous ; fin-fulcra minute or 
absent. Aspidorhynchid.® (p. 415). 
No presymphysial bone ; vertebral centra 
opisthocoelous; fin-fulcra large .... Lepidosteidje (p. 440). 
Family ASPIDOEHYNCHID^E. 
Head and trunk much elongated, the snout produced, and the 
abdominal much longer than the caudal region ; tail homocercal. 
Cranial and facial bones robust and opercular apparatus complete, 
all more or less ganoid; mandibular suspensorium vertical or 
inclined forwards, but gape of mouth wide ; a distinct azygous pre¬ 
symphysial bone present in mandible; marginal teeth slender, 
conical. Branchiostegal rays numerous. Vertebral centra annular 
or amphicoelous. Fins small, with broad flattened rays, branched 
and articulated distally; fulcra minute or absent. Scales rhombic, 
much deepened on the flank. 
Synopsis of Genera. 
Rostrum much produced in advance of man¬ 
dibular symphysis; an intercalary cheek- 
plate between the suborbitals and pre- 
operculum ; foremost scales of lateral line 
not deeper than those immediately beneath 
Rostrum scarcely if at all produced in advance 
of mandibular symphysis; suborbitals in 
contact with preoperculum ; all scales of 
lateral line deeper than those immediately 
beneath ... 
Aspidorhynchus (p. 416). 
Belonostomus (p. 428). 
