Bluish calculi with a smooth glistening surface and lower specific gravity—1681—, have been found of small size and in great numbers in the colon (1000 in the colon, Gurlt). In some calculi there is a large admixture of alimentary matters, and a low specific gravity (1605 to 1674). These were designated as _pseudo calculi_, by Fürstenberg. In still other cases a calculous looking mass, when broken into, is found to be composed of a mass of dried alimentary matter enclosed in a thin layer of lime salts. These have a low specific gravity (1446 to 1566) and have been named _concretions_ by Fürstenberg. _Causes._ As a large proportion of the calculus is phosphate of lime or ammonio-magnesian phosphate, we must look for the source of these in the food and then at the conditions which determine their precipitation. The percentage of ash and of phosphoric acid in the common foods of horses may be seen in the following table: PO_{5} in the PO_{5} in the Ash. Ash. entire food. Per cent. Per cent. Per cent. Wheat bran 7.3 50 3.65 Wheat grain 3.0 46.38 1.3914 Oats grain 2.50 26.5 0.6625 Barley grain 3.10 39.9 1.2276 Bean grain 3.10 31.6 0.9864 Pea grain 2.75 34.8 0.957 Tare grain 3.00 36.2 1.086 Indian corn grain 1.5 Rye grain 1.6 39.9 1.0384 The source of the magnesia may be found to a large extent in the grains represented in the following table: Ash. Mg. in Ash. Per cent. Per cent. Oat, grain 2.50 7.3 Barley, grain 3.10 8.5 Rye, grain 1.6 2.4 Wheat, grain 2.12 9.98 Wheat, bran 7.3 11.2 Bean 3.1 6.6 Pea 2.7 5.6 The amount of magnesia in each of these grains is amply sufficient to furnish the material for the constant growth of a calculus. Wheat bran is preëminent in the amount of its magnesia and therefore wheat bran has been charged with predisposing to calculi. In the perisperm as a whole, Fürstenberg found 1 per cent. of phosphate of magnesia, and in coarse bran not less than 2.5 per cent.