68 PALEOZOIC FISHES OF NORTH AMERICA. 
which it is done by the higher animal, but where that food consisted of 
mollusks or crustaceans inclosed in shells it was necessary that these shells 
should be crushed, and the fragments, perhaps in part rejected, in part 
reduced to such size that they could be conveniently and harmlessly swal- 
lowed. All the marine living fishes which have pavement teeth are sup- 
posed to be carnivorous, and in many of the fossil fish teeth we find evi- 
dences of much local wear, showing that mollusks with somewhat strong 
shells were brought to the point where they could be operated upon with 
the greatest mechanical advantage, and were there crushed, as nuts are 
cracked by pigs and other animals We find, too, many devices for holding 
in position the object to be crushed. Generally the individual teeth are 
blunt-pointed, projecting above the surface in such a way as to retain the food 
in place. In the dentition composed of flatter and smoother teeth, as in 
Psammodus, Deltodus, etc., the same object is less perfectly accomplished by 
a peculiar roughening of the surface by small pores; while in Archeobatis, 
as in the modern Rhyncobatus, the surface of the teeth is roughened by a 
beautiful transverse corrugation. From these essentially flat teeth the first 
departure is made in the teeth with arched or ridged surfaces of Trigonodus, 
Deltodus, Sandalodus, ete., with which those now described should probably 
be grouped as having similar forms and functions. Next come Helodus, 
Chomatodus, and Orodus with teeth which are still blunt, but have points 
or ridges projecting sufficiently from the general surface to afford a firm 
hold of softer and more slippery substances. From these blunt-pointed 
and ridged teeth the transition is easy to the sharp-pointed piercing teeth 
of Cladodus, Hybodus, and Lamna, admirably adapted to catching and hold- 
ing the most slippery and evasive prey; or to Chomatodus and the blunt 
species of Polyrhizodus, of which the teeth in form and function resemble those 
of the Rays; thence on to the teeth of the Petalodonts, which, with the 
increasing sharpness and elevation of their cutting edges, lead to the terri- 
ble, serrated, lance-pointed blades of Carcharodon. 
The affinities of Goniodus can at present hardly be conjectured. Some 
of the smaller teeth of Pyctodus, so common in the Hamilton rocks of Iowa, 
exhibit considerable resemblance to these; but all the teeth of Ptyctodus 
show something of the peculiar transverse striation of the flattened crown 
