416 
ACTIXOPTERYGII. 
Genus ASPIDORHYNCHUS, Agassiz. 
[Poiss. Foss. yoI. ii. pt. i. 1833, p. 14.] 
Bostrum slender, much produced iu advance of mandibular sym¬ 
physis ; circumorbital plates very small, suborbitals large, and an 
intercalary plate between the latter and the preoperculum ; teeth 
irregular in size, largest on the premaxilla, palatine, and presym- 
physial bone, reduced to a fine granulation on the inner face of the 
ectopterygoid; branchiostegal rays short and broad, and gular plate 
apparently absent. Vertebral centra annular. Fulcra wanting on 
paired fins, minute on median fins. Pelvic fins situated at about 
the middle of the trunk; dorsal and anal fins short-based, tri¬ 
angular, remote and opposed; caudal fin symmetrically forked. 
Scales robust, smooth or rugose; in three deepened series on the 
flank of the abdominal region, and the foremost scales of the series 
traversed by the lateral line not deeper than the series below. 
The cbondrocranium of Aspiclorhynchvs must have been consider¬ 
ably ossified, but its characters are as yet unknown. The cranial 
roof-bones form a continuous shield, and the frontals constitute 
much the largest portion of it; the whole of the roof is more or 
less rugose externally, and the median suture between the frontals 
is remarkably wavy. The paraspbenoid is narrow and delicate, 
parallel with the hinder portion of the cranial roof, and apparently 
destitute of teeth. The elongated though robust vomers meet in 
an acute angle below the mesethmoid, and are firmly fused with 
the latter ; they are likewise toothless, so far as the present writer 
has been able to observe them. The rostrum appears to be simple, 
but is often marked with longitudinal ridges which may readily be 
mistaken for separate elements in crushed specimens. The hyo- 
mandibular is much expanded, with a large process for the support 
of the operculum; and immediately behind the fan-shaped quadrate 
element there is a small narrow symplectic, widest at its upper end. 
The ectopterygoid is toothless behind, where it exhibits a moderately 
deep expansion, but bears further forwards a closely-set series of 
slender teeth, which gradually increase in size to its anterior ex¬ 
tremity where it articulates with the palatine. The teeth on the 
latter element are largest behind and diminish forwards. The 
entopterygoid is long and narrow, while both this and the im¬ 
perfectly-known metapterygoid are delicate. The epihyal is small 
and the ceratohyal very large, but the basihyal is unknown. The 
maxilla is much elongated and slender, bearing a series of relatively 
small teeth; and there is a curious laminar expansion along its 
upper border immediately in advance of the orbit. There are also 
