It is well known that the crushing teeth of the Pycnodont fishes as a rule are arranged with striking regularity upon the bones which bear them, the vomer in the upper jaw and the splenials in the lower jaw. They are disposed in longitudinal series, gradually increasing in size backwards, and there is only one genus satisfactorily known (Mesodon) in which there is any marked tendency towards the frequent disturbance of this linear arrangement.