Davis—On the Fossil Fishes of the Chalk of Mount Lebanon. 495 
Sub-order.—PycnoponTorDEI. 
‘Body compressed, high and short, or oval, covered with rhombic scales 
arranged in decussating pleurolepidal lines. Notochord persistent. Paired fins 
without axial skeleton. Teeth on the palate and hinder part of the lower jaw 
molar-like. Branchiostegals, but no gular plates.” (Giinther.) 
This sub-order comprises two families, the Pleurolepid, in which the body is 
not high and the fins have fulera, and includes two species from the Lias; and 
the Pycnodontide, which is more or less rotund in form, and is especially 
distinguished by the peculiar form of its teeth. 
Family. PYCNODONTIDZ. Aaassiz. 
‘‘ Homocercal. The neural arches and ribs are ossified ; the roots of the ribs 
are but little expanded in the older genera, but enlarged in the tertiary forms 
so as to simulate vertebrae. Paired fins not lobate. Obtuse teeth on the palate 
and the sides of the mandible; maxilla toothless (rows of teeth on the vomer); 
incisor-like teeth in the intermaxillary and front of the mandible. Fulcra absent 
in all the fins.” (Giinther.) 
The Pycnodontidz were recognized by Professor Agassiz and formed into a 
family, which was distinguished by the arrangement and form of the teeth. 
(Rech. sur les Poissons Fossiles,” vol. 1., part 2, p. 184.) Three genera were 
placed by Agassiz in the family, Pyenodus, Microdon, and Gyrodus, and to these 
must be added, amongst others, that of Palaobalistum, de Blainville. They are 
abundant in the secondary rocks, and extend upwards to those of the tertiary 
period, but are now extinct. The teeth of Pycnodus vary in form and size, from 
small circular teeth on the rami of the jaws, to large, oval, or elongated forms on 
the palate; those of Gyrodus have a circular groove parallel with the margin of the 
tooth, whilst the teeth of Microdon are small, subcircular or oval, and do not 
vary in size as those of the other genera do. In the upper jaw there is a row of 
teeth on each of the maxillaries, with five rows on the vomer. On the lower jaw 
there are four rows on each side. The anatomical structure of Microdon is, 
perhaps, best known; specimens of this genus have been found well preserved, 
whilst those of Pycnodus and Gyrodus are often represented only by their teeth. 
Sir Philip Egerton indicated a peculiar feature in the structure of the scales 
of the Pycnodonts (‘‘ Quart. Journ. Geol. Society,” vol. v., p. 331): ‘Each scale 
bears upon its inner anterior margin a thick solid bony rib, extending upwards 
beyond the margin of the scale, and sliced off obliquely above and below, on 
opposite sides, for forming splices with the corresponding processes of the 
adjoining scales. These splices are so closely adjusted that, without a magnifying 
power or an accidental dislocation, they are not perceptible. When 7 situ and 
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