Micro Teeth from the Potomac

The last few times I've gone to the Potomac, I've brought home some gravel, hoping to find some of the rare micro teeth.  I built a sifter bow that has 1/4 inch mesh on the top, and window screen on the bottom.  The material that falls through the mesh but is trapped above the screen is dried and carefully searched.  The photos show the better finds from two 1-quart baggies of processed gravel.  Not shown are the hundreds of fish and ray teeth, juvenile teeth of common species, and unidentifiable stuff.  Most of these teeth are in the 2-4 mm size range.