518 
DR. A. GUNTHER'S DESCRIPTION OF CERATODUS. 
Dentition. 
The dentition is essentially that of Lepidosiren, and resembles that of certain extinct 
genera (Dipt er us, Clieirodus , Conchodus, Psammodus , &c.), as we shall see hereafter. 
It consists of a pair of vomerine teeth, and of a pair of maxillary and mandibulary dental 
plates ; but whilst the dentition of Lepidosiren is chiefly adapted for piercing and cut- 
ting, that of Ceratodus is modified for the functions of cutting and crushing. The 
vomerine teeth (Plate XXXIY. fig. 3, v, and Plate XXXV. fig. 1, v') are broad and rather 
low laminfe with a convex and trenchant margin, the outer or posterior part of which is 
slightly serrated. Each lamina is 13 millims. long, and in the middle 5 millims. deep. 
They are inserted in an oblique direction to the longitudinal axis of the vomer, and meet 
in the middle at a right angle ; being implanted in cartilage, they are slightly moveable. 
Each maxillary dental plate (Plate XXXIV. fig. 3, and Plate XXXV. figs. 1 & 2 ) is an 
oblong piece with a grinding-surface, a convex inner side, and with the outer side divided 
into six prominent trenchant ridges or prongs by five notches, of which the foremost is the 
deepest, the others becoming shallower posteriorly. The foremost ridge passes to the 
inner border of the tooth, which is likewise somewhat raised. The grinding-surface 
has a great number of minute depressions (punctuations). The total length of a maxillary 
tooth is 32 millims. ( = 1|- inch), and its greatest width 13 millims. (=5 inch). In form 
and size the mandibulary teeth (Plate XXXV. figs. 1-3) are very similar to the maxil- 
lary ; only the grinding-surface is less uneven. These teeth are anchylosed to the bone, 
and inserted in an oblique direction : the upper teeth nearly meet each other in the 
median line ; but there is rather a wide interspace between the lower. The double kind 
of action which these teeth have to perform may be easily understood when the mouth 
is closed. The fiat surfaces of the upper and lower molars are opposed to each other, 
and serve for crushing or grinding food, whilst the sharp lateral ridges of one tooth fit 
into the notches of the opposite tooth (Plate XXXV. fig. 1) like the shells of a Cardium, 
this part of the arrangement being adapted for cutting. The foremost ridges of the upper' 
molars are received in the wide space between the lower ones, the vomerine teeth being- 
opposed to the concave dilatation of the symphysiai part of the lower jaw. 
I 11 a vertical section of one of the grinders (Plate XXXII. fig. 1) it is seen that the 
real depth of the tooth (that is, of that portion which is formed by dentine) is much less 
than it appears from a merely outward inspection. It rests, in fact, on an elevated 
plateau of the dentary bone (fig. 1, c), which has exactly the same outlines as the tooth 
itself, and the substance of which passes so gradually into that of the tooth that it is 
only by the difference in the shade of colour that the boundary between osseous 
base and dentinal crown is indicated. This anchylosis, however, is limited to the cir- 
cumference of the base of the tooth ; for its central parts are separated from the bone by 
the extensive but shallow pulp-cavity (fig. 1 , h). We must remember that our speci- 
mens of living Ceratodus are by no means aged individuals, certainly much smaller and 
younger than those gigantic individuals of extinct species must have been, of which 
teeth 2 and more inches long are preserved. In such fossil teeth no pulp-cavity is 
