128 
ACTIXOPTEKYGII. 
Genus DAPEDIUS, Leach ( emend . Agassiz). 
[Trans. Geol. Soc. (2) vol. i. 1822, p. 45 ( Dapedium ), and 
Agassiz, Poiss. Loss. vol. ii. pt. i. 1835, p. 181.] 
Syn. Tetraqonolepis , L. Agassiz (non Bronn), Poiss. Foss. vol. ii. pt. i. 
1833-35, pp. 6, 181. 
Amblyurus, L. Agassiz, ibid. 1836, p. 220. 
JEchmodus , Sir P. Egerton, Quart. Journ. Geol. Soc. vol. x. 1854, 
p. 367. 
Omalopleurus, 0. G. Costa, Ittiol. Foss. Ital. 1873, p. 59. 
Trunk much laterally compressed, cycloidal or very deeply fusi¬ 
form. Head comparatively small, with well-developed opercular 
bones arranged in an arched series; operculum deeper than the 
suboperculum ; preoperculum narrow and almost or completely 
covered by the suborbitals, but all the other external bones more 
or less ornamented with tuberculations of ganoine; a large gular 
plate present. Teeth robust, styliform, with simple, bifid, or mamilli- 
form apex, arranged in clusters within the mouth. Notochord 
persistent; ribs ossified. Fin-fulcra large and uniserial. Paired 
fins small, and the pectorals situated well upon the flank; dorsal 
fin much elongated, arising about the middle of the back, and anal 
fin shorter, opposed to the hinder half of the dorsal: caudal fin 
slightly forked. Scales quadrangular, robust, ornamented with 
tubercles dorsally and ventrally, nearly smooth or similarly orna¬ 
mented on the flanks, and united by a peg-and-socket articulation 
at a slight rib-like thickening anteriorly ; scales of the flank deeper 
than broad, the others nearly equilateral, and the dorsal and ventral 
ridge-series inconspicuous. 
The chondrocranium of Dapeclius is well ossified, and there seems 
to have been a complete, or nearly complete, interorbital septum. 
As seen in side view, the basicranial axis is sharply bent upwards 
in front of the otic region; and the basioccipital element is deep, 
much excavated behind for the notochord, and longitudinally 
grooved below for the basicranial canal. The divisions between 
the elements of the cranium and the situation of the foramina for 
the nerves have unfortunately not hitherto been distinguished ; but 
it is clear that there was an ossified supraoccipital, with a vertical 
median ridge behind, and there are robust ossifications in the pre¬ 
frontal and postfrontal regions. The ethmoidal region terminates 
in front in a small blunt process, pierced transversely by a large 
foramen; it expands on each side, in advance of and below the pre¬ 
frontal, into a great mass sheathed by the vomer. The olfactory 
nerve evidently passed through a forameD observable between the 
