PYCXODOXXIDJE. 
197 
tion of the tritoral teeth to a fixed arrangement in regular longi¬ 
tudinal series ; and (in.) in the obliteration of the indent for the 
meckelian cartilage on the attached face of the splenial bone. Very 
rarely ( Coccodus , Xenopholis) a spinous dermal armour is also 
developed. Finally, we venture to recognize one case of degeneracy 
of the tritoral dentition ( Anomoeodus ). 
Synopsis of Genera. 
I. Neural and haemal arches of axial skeleton of 
trunk not expanding sufficiently to encircle 
the notochord. 
(a.) Number of longitudinal series of teeth 
on splenial bone inconstant; caudal fin 
rounded ; no dermal spines. 
Vomerine teeth in five longitudinal series, 
sometimes irregular laterally ; splenial 
dentition comprising one regular 
principal series, with three or more" - '' 
outer series and one or two inner 
series, usually irregularly arranged; 
scales tuberculated, confined to region 
in advance of median fins. Mesodon (p. 199). 
[Splenial dentition as in Mesodon, but 
usually much more irregularly ar¬ 
ranged, and the principal longitudinal 
series not well - differentiated; the 
splenial bone niuch stouter, and with a 
remarkably deep symphysial facette; 
fish unknown . Athrodon (p. 215).] 
Vomerine teeth in three regular longitu¬ 
dinal series, with irregular interme¬ 
diate series; splenial dentition with 
two regular series separated by small 
teeth, and flanked within by other 
small teeth ; scales covering whole of 
trunk, united above and below with 
jagged sutures. Mesturus (p. 218). 
( 1 .) Number of longitudinal series of teeth 
on splenial bone constant; caudal fin 
forked ; no dermal spines. 
Vomerine teeth in five longitudinal series, 
the inner lateral pairs regularly alter¬ 
nating with the widely-spaced median 
teeth; splenial teeth in four regular 
series ; squamation confined almost or 
completely to region in advance of 
median fins. Microdon, p. 221). 
